In my bathroom I've put vinyl flooring on top of some sort of underlay but the vinyl is moving and creasing a lot:
Here's what is underneath, a blue underlay and then the wooden floor under that:
What's the quickest way to make this vinyl stable? What about stapling it? In this case, how many millimeters extra (beyond thickness of the vinyl itself) are required?
Staple down the underlay, and then I'd go with @mike's suggestion of double-sided carpet tape.
Installing molding around the edges will also help to hold it down and prevent buckling, especially if it's relatively thick (eg, includes quarter-round) and tight to the floor.
Normally vinyl is just glued down. Robert's adhesive can be bought at the big box. Loctite makes glue for vinyl that can be bought in a caulking tube. You just need to lay some heavy things on the floor after you glue and let it sit for a half day or so.
