+1

You do have a path to refactor the database and not change a line of VB6 code. It would involve refactoring the tables that currently contain multiple entities (Entity and EntityType, dead giveawy on the naming :-D) and then creating VIEWs with INSTEAD TRIGGERS on them to handle the create, update and delete operations in your now-normalized base tables. Your VB6 will not know the difference.

As for CONSTRAINT or TRIGGER, pick your poison. They'll both be a challenge to develop, test and support.