i'd say find the lightest client possible. Java is lighter than flash,but ajax is even lighter. A friend wrote a chat client in ajax,that integrates well with the forum (show how many users are in forum in the chat, integrate user accounts from forum in the chat)... the only con is that it doesnt use irc,so it cant be bridged with vecs irc channel.http://teli.no-ip.org/ajax/client.html
*this demo is not bridged to the forum.to see a fuller version,check garbagewars.com/forum -->chat
*its on a slower server,so it might load pretty slow
*the server that it is hosted is a home vector server. My friend is willing to put it at google code as gpl, its extremely simple and light...but it has a few things to iron out or add.