The integration of Bamboo and EC2 has more to do with dynamically allocating instances as you need them -- elasticness. So this is wound pretty tight with the Amazon AWS API. I'm not aware of vCloud support being planned. You can have a look through jira.atlassian.com to confirm.
At the same time, though, virtual instances (no matter where you host them) can also be treated as just normal remote agents. You'll have to manage the agent processes inside the instances yourself but that's fairly straightforward to do and can scale quite a bit. That's of course assuming you can also setup a secure channel between your vCloud instances and your Bamboo server instance to allow the Bamboo traffic.