Is it possible to move only selected albums to a vault?

No. A vault backs up the Aperture library and all photos contained within.

Why do you want to move just one album/project to the vault?

Tony

You could work around the limitation by selecting the projects/albums you want to turn into a vault, exporting those out as a new library, then open up the library to create a vault from it, although you wouldn't be able to restore that vault back into your main library.