I didn't check this from system, so I might be wrong, but isn't the address number the "key" here? I mean that when you create a new vendor/customer with an address, the address is stored to the central address table (ADRC?) with an address number, and the address number is then stored in some of the main tables of the vendor/customer (perhaps even the header table of the corresponding master data object, check table LFA1 for address field). This should be easy to check. If this is true, then here is your link. So, theoretically e.g. in the check function you could check if you find the address number in the vendor table, and if yes, then you will know that you are changing a vendor address.
Not sure though if the above way is the best possible way. Might have been easier to locate to a possible user-exit/BADI for vendor change (unless you did already).
Regards,
Karri
PS. What's the Address.IdType? Could this have a different value depending on if it is vendor, or customer or whatever?