When installing vinyl floors, it is highly recommended to use a moisture barrier. This is true even if the vinyl floor has an attached underlayer. The moisture barrier will help protect the floor from moisture and other elements. It is also necessary to install a moisture barrier if you are using a separate bottom layer, such as when covering existing hardwood floors, vinyl floors that aren't padded, or concrete floors.
Just make sure that any undercoats you add are thin, so that they don't affect the locking process of the vinyl boards. To add more comfort and cushioning benefits, it's best to place a bottom layer under a waterproof vinyl floor. Once you have a smooth surface to work with, you'll have to decide if you want to install a subfloor. Generally, a lot of people prefer vinyl floors with attached subfloor, as this is likely to be a more affordable option rather than buying vinyl flooring and a separate underlayer. If you are installing vinyl floors over wooden subfloors, you won't have to worry about a moisture barrier.
However, you may want a base layer for added cushioning or sound reduction. It may also be a requirement in certain HOAs or apartment complexes to have an acoustic barrier on the floor. When installing it on a concrete subfloor, keep in mind the fact that it's a very hard surface, so you might want a subfloor with some extra cushioning. I have a California C-15 and C-54 state contractor license for the installation of floors and tiles. In general, it is important to use a moisture barrier when installing vinyl flooring. This will help protect the floor from moisture and other elements.
It is also necessary to install an undercoat underneath the vinyl floor for added comfort and cushioning benefits.
