I have laminate floor, but some of it has buckled from moisture. If I sand these areas down, will I be able to lay the vinyl flooring over the laminate?
You are setting yourself up for trouble if you take this approach. Any buckles you see in the laminate floor, even if sanded, will continue to worsen. Even more, buckling laminate is often the sign of a moisture problem, or improper installation, or both!
Laminate floors are really easy to remove because they're floating - that is, they're not glued down. I definitely recommend removing it completely and doing your best to control the moisture before considering any new floor. Plus vinyl needs to be installed on a clean, smooth and completely flat surface or every imperfection of the old floor will show through.