re: Vinyl plank flooring over concrete

If it is glue down, like stated numerous times you don't need a vapor barrier. Just make sure slab is super clean and perfectly smooth or you will have issues. If it is a locking LVT (one that clicks together like laminate a vapor barrier is also not necessary. IF you use an underlayment for sound, use one that is specifically designed for LVT. If you use a laminate underlayment it could make the locking system disengage and your floor with look like shite.