(just an update, As a workaround for IE on Vista issue, Apps 11.5.10 works fine on Vista with firefox)
Our network department has a plan to go with windows vista and they asked me to open a TAR with oracle to get their official response.
Below is oracle response
Summarize customer's question here. One question per heading.
Unable to connect from MS-Vista Client on e-Business suite 11.5.10
Currently Jinitiator is not certified on MS-VistaDevelepemt is working on it but we don't have any ETA for it So, please don't upgrade to MS-Vista Sorry for that
Platform / Port Specific? = Not certified for the moment