should be:
vm.$set(vm.userData, 'sp', totalSteemPower.toFixed(3))
vm.$set(vm.userData, 'vesting_shares', vestingShares.toFixed(3))

not
vm.$set(obj, 'sp', totalSteemPower.toFixed(3))
vm.$set(obj, 'vesting_shares', vestingShares.toFixed(3))

I think. It errors out with obj and works with vm.userData. Unless there is a reason not to use vm.userData