Well, I wouldn't use just vaseline. I heard of someone using vaseline with plastic wrap. They coated the surface with vaseline, then layed the plastic wrap on top of it. The vaseline just kinda held the plastic wrap in place. However, I imagine it would be hell to clean up all that greasy schtuff. You should just use tinfoil and tape... that's probably your best bet.
Check out this site really good step by step with pictures:
http://members.iinet.net.au/~nismo/subs.html
Here's a pic of my sub box that I'm working on, although it isn't a "sub tub"...
