I replaced the transmission fluid on my 2003 with 96,000 miles on it and noticed a significant improvement in how the vehicle performs. The fluid that I took out looked way too thin and was black as engine oil. I noticed a significant improvement in my mileage after making the change.
Vibes require transmission fluid that is Toyota T4 rated (also sometimes called "Delco T4"). I skipped the $30 per quart Toyota/Lexus brand stuff and got Valvoline Maxlife instead ($8.49 a quart where I bought it) and it seems to have worked just fine.
There's a mechanic who posted an awesome video on Youtube about servicing your transmission. You don't have to drop the transmission oil pan like he does though, just pulling the drain plug and letting it set for half an hour should work fine.
http://www.youtube.com/watch?v=En9pE78Wifk" onclick="window.open(this.href);return false;