Speaking slowly is a good way to learn Vietnamese. Vietnamese is monosyllabic language and speed to speak is slow. You mustn't speak fast. Only speak slow each word.
For example: "Tôi là một công nhân". Let pronounce each word from 3-5 s. Tôi/là/một/công/nhân :D
Then record and listen again. If you still speak too fast, you can change it. And repeat, repeat, repeat many times. You will see your speaking skills that will be better.
A German linguistic said: should to learn Vietnamese by speaking first, not passive listening. Should to pronounce each word. It seems to be boring. Actually, it's very useful to learn Vietnamese. ( You can visit the link below)
I knew my Indonesian friend who feels fear when he listened to my voice about "A" with 6 tones in Vietnamese.
After you will be good at each vowel, you should speak phrases, sentences simply like name, job, age, address...
Finally, don't forget to research Vietnamese culture to understand Vietnamese better.
Visit this site to learn more:
http://www.kanji.org/kanji/jack/vietnamese/is_VN_hard_sum_EN_VN.pdf