Hi,
vCAC (vCloud Automation Center) is strategic product as far as I know. vCAC is focused towards self-service provisioned and have lots of plans ahead.
However It is still not multi-tenant aware. This is big drawbag of this product. Being said that I don't know if I have answered your question.
If you are looking for self-signup website you will have to depend on API for it.
Thanks,
Preetam