So, Sony Vegas 7 is leaking memory all over the place.

I have a more-than 20 minute LFS movie on the go, and when I play through it my RAM usage goes from about 50% to around 90%, gradually. This figure doesn't decrease until I exit Vegas and restart it.

Is there anything I can do to improve this?

Get a better computer : ) no in all seriousness, that sucks cause it makes it a pain for you... Hopefully someone knows, but I wouldn't count on it.

..maybe try to close all other programs.. media player, msn, sidebar (if u have vista..)

have you tried to render the movie by parts? Vegas uses less Ram on playing an already rendered movie than an actuall preview