Ally them. Call them to war against someone they won't join, or someone that they have truce, when you get mission to conquer/vassalize them.
Why bother? Just vassalize them. They aren't strong enough to take on Muscowy.
Librerum Veto is god tier decision : It only cost you 1 centralization move, gives you +10% army tradition, 2 aristocracy, 15k troops, and reduce WE by 8 points, also allows you to get event called "Reform" which gives +1 base tax in 3 random provinces, gives you 3 moves to centralization, and 3 moves to plutocracy - and it just need ruler with at least 6 in each stats(You may be republic, and this makes it super easy to get if you are). Also, it gives you event that spawns randomly 3k troops and reduce your WE by 1 point.
It have also bad event, but i say that it is still good enough to take.
I've never gotten either of those two events.