What version of Vista are you using?
And what have you tried to do to fix the problems? There is a way to get vista and simplemu to get along. I've been using it on two vista machines for awhile now and have had no problems other than a funky install to weird place and not find it the first time I loaded it onto the one machine. Do you run it as an administrator? That works for a lot of people.
Otherwise if you scroll through this thread: http://www.gammon.com.au/forum/?id=7844
there are suggestions offered for fixing the issue.
I have, as you can see, no suggestions on other mush clients. I bought simplemu awhile ago and have never tried anything else. Good luck!