Actually, I've found more issues with my speeds on uTorrent than Vuze. (Beyond the silly memory use issue which isn't even an issue unless your computer is a piece)
Vuze can grab me faster speeds with 4+ torrents. No matter how many torrents I have open, I get my "max" of 60kb/s upload. (Dl doesn't seem to have an issue) However, with 4 or more open in utorrent, I get each of them running at 2-6kb/s or about 30. Imo, it's not a connection issue for myself, but a software issue. I'm considering going back to vuze when I reinstall windows 7.