and just to check.
you went to the policy screen
the groups tab
and clicked on vcenter and EVERY folder below to verify there are not groups anywhere,
and went to datasets and verified there are not dataset set in there (with the off chance some other group is using a dataset in that domain)
Just trying to think of every place to check.
How are you?. Hope you are doing good.
Try checking all the disabled and enabled backup groups if they are residing under vcenter domain. There should be some backup groups except "default" group, existing under that domain. Delete that group and try deleting vcenter. Basically vcenter give error if it has any sort of mapping from Avamar Database perspective.