Can't advise re: Outlook on Vista machine - I stayed w/ XP after hearing so many Vista complaints.
Re: syncing BES w/ Outlook on more than one computer ... I don't think it should be a problem, but that's a question best asked to BES host tech.
Re: not using Outlook for email ... if you auto-forward your Yahoo/Gmail incoming messages to the email address for your hosted BES account, those forwarded messages will show up in Outlook. You can process them there or only process it on your BlackBerry.
I'm not an expert on this, but I don't think you can keep your Yahoo or Gmail mailboxes in sync with your hosted BES account. If someone knows of a way to do so and shares it, I may learn something new. I only process mail in my BES mailbox. With all mail forwarded there, I can't remember the last time I actually opened any of my other mailboxes.
Hope this helps.