I think you need to not think of IPB "hosting" as a regular web host, and instead consider them a software services company. When you get a hosted IPB instance from Invision, what you are really getting is a subscription to a software service, in this case the IPB forum. I would really doubt that IPB would be interested in just being a regular web host that anything could be installed on, Vbulletin or whatever.
Would you go to salesforce.com, or Basecamp, and ask them to host a Vbulletin forum? No, because they are not "hosts", they are software services companies. I don't think Invision is any different.