It's really not any different than your opponent bribing a powerful ally against you, and that's fine, I guess?. The vassal state offer is simply a good offer to an AI.
It's also realistic, I would argue. There may be plenty of reasons why a powerful civilization doesn't want another one getting more powerful, and so choose to support weak victims of other civs.
It's also realistic, I would argue. There may be plenty of reasons why a powerful civilization doesn't want another one getting more powerful, and so choose to support weak victims of other civs.