if my opinion counts, I would like to suggest staying with Jabber. The clients can be customized easily and running a jabberd is easy as well. Java clients as you said cannot be customized a lot in their look and feel.
PLUS .. there is an option to churn out an integrated IM client which does vbulletin messaging as well as others (AOL, MSN, etc). The open source clients already do all that. We just need to provide back end connectivity to VB.
just my 2 pennies ..
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