Voluntaryism implies anarchism, and is essentially synonymous
If you are a voluntaryist, you oppose the legitimacy of all forms of coercive (aggression initiating) interactions. In this framing, the State is a set of coercive interactions (typically performed by a known pre-defined group of agents or actors) that are considered to be legitimate by some people (which may or may not include some of the victims). Voluntaryists oppose ALL coercive actions -- including that of the State. Purely theoretically, one can be an anarchist, but not a voluntaryist generally -- but, that would be extremely doubly odd. It's like saying 'I oppose State aggression (that some people believe is legitimate); but, I support it if it is private aggression, like that of a private gang or terrorist group (that no one else believes is legitiamte)'. So, practically speaking, voluntaryism and anarchism are synonymous.
Footnote : 'volunteerism' and 'voluntarism' are terms unrelated to 'voluntaryism' and to each other.
Framed in the light of recent history and contemporary terminology
With the Austrian economics revolution of deductive economics (based on praxeology and methodological individualism) of the late 19th and early 20th century (Mises, Hayek et al.) and in its later full synthesis with individual anarchism by Rothbard and then his students in the late 20th and early 21st centuries leading to the body of work sometimes generally categorized under the umbrella of 'anarcho-capitalism', modern voluntaryists or anarchists often refer to themselves as such. Used in this modern context, the word capitalism refers to the observation that man, when not coerced, engages not only in intentional productive activities to meet their needs, they also often save any extra produce for later use, or to convert into tools or new skills, or for trade and joint entrepreneurial projects with other people -- generally referred to as free market activities. The hyphenation -capitalism is added to emphasize that anarchists are anti-anti-capitalist, given that capitalism, as defined above, is a natural action done by free men. It is also likely a necessary natural action -- there may not be anyone (even among the extremely risk-loving high-time-preference types) who values future moments of their life at exactly zero. Furthermore anarcho-anti-anti-capitalist or anarcho-free-marketer are mouthfuls, indicating some reasons why the phrase anarcho-capitalism (or, ancap) has usefully stuck, and grown in use.
It is important to clarify that capitalism, as defined above, has nothing to do with corporatism, which is essentially State-favored or partially or fully State-owned monopolistic cartelized or syndicated manipulations of the market. Some modern anarcho-communists (along with its many different flavors, like georgism, mutualism, etc.) emphasize their opposition to corporations and corporatism, which all anarchists share, by definition. However, sadly, many modern anarcho-communists have lost that focus and have slipped into Statisms of various kinds. More on this below.
The initial modern re-awakening of explicit anarchist thought in the 19th century arose among the anti-corporatist revolutionary Left, represented by the likes of Emma Goldman and Alexander Berkman. Very quickly they diverged, in theory and method, from Marxist Statist communists and especially Soviet Bolshevism, which began to correctly/consistently regard anarchism as yet another enemy. In the modern day (21st century), there is a small anarcho-communist movement, which can mean a variety of things. True anarchist-communism does not preclude capitalistic activities and is also inseparable from private property norms (if something is granted fully to the 'common', it is merely loaned or gifted to another person). In other words, true communism, if defined as shared ownership may be logically impossible, given that a man may be unable to divorce his will entirely from his person or even merge his will forever with another to create one unified indivisible will. One can loan or gift, but one cannot physically consent to being a slave and being owned, in full, by another. A thing is either yours or not -- it cannot be yours some of the time and not yours at other times, in a conflict-free manner. Therefore, there is no need to distinguish between anarcho-capitalism and anarcho-communism in principle, since, in a pure principled sense, they can co-exist. In any case, practically speaking, people cannot truly actually live anarchistically and communistically, in a long-term sustainable way. Communism and communists, practically speaking, require a State. Therefore, practically (but, not in principle), anarcho-commies are just Marxist fascists pretending to be anarchists. Therefore, practically speaking, the modern term 'anarcho-capitalism' helps one identify more strongly as a fully principled voluntaryist.
How does libertarianism fit into all this?
Libertarianism is a legal theory in which initiatory aggression can NEVER be justified (requiring no restitution from the aggressor) in legal judgements. Any judgements that pertain to resolving disputes (whether dispensed by direct negotiation, 3rd party arbitration, private courts or village elders, or anything in between) that is consistent with this principle may be called libertarian. It should already be clear that voluntaryists are libertarians. Now a non-voluntaryist may still prefer a libertarian legal order; however, a libertarian legal order can only exist and be sustained in a society that is both Stateless (anarchic) and predominantly voluntaryist -- hence, a rational libertarian is always anarchist/voluntaryist.
A frequent confusion and categorical conflation is the idea of 'political libertarianism'. To reiterate, libertarianism, per se, is not a political position. It is a legal theory. Political libertarianism is a hodge-podge of minarchists (mainly) and anarchists as well as a small group of elitists who seek to use the culture of politics (campaigns, fundraising and even winning elections) to either spread the message of liberty (most of the former) or spread libertine values that are either anti-libertarian or unrelated to libertarianism (the latter group). Some of them, inconsistently, also believe they can use the power of the State to lessen the power of the State -- in other words, they mistakenly believe that they can use electoral democratic systems to make the world more libertarian.
Meta-ethics, religion, etc.
Voluntaryism is an ethical position. But, ethical positions can be given a metaphysical grounding.
If one grounds their voluntaryism in the metaphysics of human rationality, one is a natural law anarchist. This tradition of grounding enables one to understand the non-aggression principle (NAP) and private property norms (the libertarian ethic) as well as polycentric law. Rothbardians fall into this category.
If one grounds their voluntaryism in the metaphysics of human nature and the impossibility of authority, then, one is an individual anarchist. Larken Rose is a chief exponent today and this is the Lysander Spooner tradition.
Christians will be delighted to learn that both groundings are consistent and complete with respect to Christian theology and ethics.
Regardless of the grounding (and either one is sufficient on its own), anarchists/voluntaryists can derive their whole ethic as well as libertarian legal theory from it.
Agorism, markets etc.
As long as a State exists, a free market does not exist. Free markets can only exist under anarchy/voluntaryism. So, what are we to do in the meantime? Agorists are folks who believe strongly in conducting as many economic transactions as possible without State theft (self-sufficiency, mutual aid, tax evasion, barter, crypto, cash-only without reporting, etc.), i.e. in the so-called gray/black markets, which are the only free markets today.
(Austrian) Economics
The Austrian economics (praxeological) tradition of Mises, Hayek, Hazlitt and others allows one to criticize many consequentialist/utilitarian fallacies that underlie Statism (i.e. the opposite of anarchism) and provide sound economic analyses and insights. These include the knowledge problem, the incomparability of inter-subjective preferences. Some (for e.g. David Friedman) have gone further and developed voluntaryism and anarcho-capitalism purely from this school of economics -- these arguments are grounded in certain consequentialist ethics based on economic analysis.
In summary, I am an anarchist (anarcho-capitalist to be redundant), voluntaryist, agorist, libertarian. It would be at least strange, if not logically inconsistent, to be one of these and not all of them. It is immoral to not be them.