Is a possibility of reverter a vested interest?

You have asked a very interesting question.

The answer is yes. A reverter clause in a deed is only a possible future interest. The grantee becomes the owner of a fee simple estate. In fact, in order to be enforceable the restraint in the deed must be reasonable. Many reverter clauses are defeated by a court action.

However, keep in mind that property that is subject to a reverter clause may be difficult to mortgage or sell.