You can only be a vassel, or be an ally. Vassel implies that you are weaker, that you have no other choice then subjecting yourself to the other Kingdom. This is in no way the relation you have with Sheb.
On the other hand, Ally, or Alliance, implies equal, or that we are at the same level. This is most certainly what you and sheb are. It would make far more sense to become allies rather than vassel and master.
And an alliance is beficial to both, rather then one-sided. In your vassage to Sheb, you must send him units no matter what if he is attacked, and there is nothing on what happens when you are invaded. In our alliance, we send all our spare units to help any faction that is attacked by an agressor, no matter what. And you must send your spare units, if you have none to spare, then you may not send any. And it is agreed that factions must help any faction that is attacked.
I am trying to prove that you want an alliance, not a vassage. An alliance would be a better lasting, more fruit-ful way to befit all. A vassage would be a stupid and regetful choice for you.
Edit: I am also less worried by the relations we have with now, then the relations we WILL have with him. It is impossible to stay out of world politics forever, and Sheb may turn up on the wrong end of our weapons in the future.