I'd say include just the last post for the vtp feed... but since I'm sure many people would start to use the feed for "offline" reading, maybe having all the posts in the thread as an optional second feed? Not sure... it really depends on the amount of bandwidth you are willing to use for it.
Speaking of "offline" reading, how about a "full text" feed for the front page as well?
By the way, the
feed is down...
For a bit more information about RSS feeds,
daringfireball originally turned me onto the idea (search the archives) and has some great comments about it.
The latest post touches on how full-text RSS feeds have been driving his membership.