Should I force vassalise my historical rival?

Solution 1:

It's easier to conquer a single Syrian core and release them as a vassal. 50% liberty desire is a HUGE penalty. It's not worth it IMO

Solution 2:

There is also a financial motivation against it. Can I assume you have kicked them quite far to the east by now? In that case their capital is likely no longer in the Constantinople trade node. Feeding them provinces will mean that much more of the trade flowing from the East will get stuck in Aleppo.

I would just take the provinces, get the trade flowing and use the extra cash to hire better admin advisers. That way there's no need to take the long route.

In summary, no I also don't think it's worth vassilising them.