Open Vegas, add your video clip to a timeline, then add a mask above that. Render to a new file. Use the new file in your DVD project. Or just menu background at the very bottom and render the whole thing to a new motion background.
If you're really set on making a "button" that doesn't lead anywhere, then set the Navigation properties for the rest of the buttons on the menu so that none of them lead to the one you're trying to pretend isn't a button. ;-) Understand that this will accomplish what you want in a set-top player, but anyone using a software-based player will be able to select that button with their mouse. If you must, simply set the end action of that button to link back to the same menu.