sounds like a pretty common vegas problem, if you don't have a decent system it will usually react with preview lag after a few saves, closes, and reopens. nothing else can really cause it, except maybe xvid/divx footage, but even that doesn't all the time, that will most likely just result in random absence in the footage replaced with black/red solids. my video preview still lags and i have over 1400mb of RAM, but even with 500 it never lagged that intensely.
maybe closing all other background operations you're using could help. also, you could try restarting your computer.
but no, switching to premiere or after effects won't have much of an alternate result. it will most likely remain to lag. (also, just to add: after effects is much different than sony vegas and adobe premiere. vegas and premiere are more meant for prime editing (like basic cuts, effects, etc.), and after effects is more useful for taking the renders from those two, and adding effects that aren't to simple to produce and finishing them off as a final export.)