This a good question as many underlayment come with built in vapor barrier and some don’t. Depends on your subfloor type also.
Why do you need a moisture barrier in the first place? A moisture barrier blocks moisture from reaching your floor. Blocking moisture eliminates havoc (warping, etc) to your floor.
How does moisture get to your flooring? Mostly concrete subfloors transmit moisture. Concrete is porous and allows moisture to pass through to your basement/room level.
So if you have a concrete slab/subfloor, then read on. If you don’t, you won’t have to worry about a separate moisture barrier as plywood subfloor doesn’t transmit that much moisture.
Knowing how much moisture is being emitted by your concrete slab is the first step. Using the calcium chloride test, you will be able to calculate how much lbs / 1000 ft² / 24 hr. per ASTM E 96.
After getting this number, you can then check what your underlayment can handle as for lbs / 1000 ft² / 24 hr.
If the amount is more than what your underlayment can handle, then yes you will need a moisture barrier. If it doesn’t exceed the amount, then the underlayment can handle the job.