You are able to buy a single VIP "ticket" by going to the "travel packages" page under the tickets section.
If you buy a GA ticket, then you can click on the VIP Camping + Shuttle package to upgrade your GA ticket to a VIP one. Works out to be roughly the same price. I spoke to Bonnaroo a few weeks ago about this and you do not have to even use the shuttle, if you just use it as a single VIP upgrade that's fine.
Guess it's kinda complicated but I can't imagine splitting a VIP package with an Inforoo stranger would be any less complicated.