Coring things yourself + vassal feeding > vassal feeding only.
And coring things yourself is likely, in turn, to imply converting things yourself. (Unless you have Humanist-synergizing NIs or religious values, in which case Humanist > hypothetical Religious-without-DV.)
I don't see how that makes it anymore "broken" than vassalizing a tag that already takes religious. It might actually make people use marches. I've done WC (one tag and domination), and have never used a march once, or kept a specific vassals for longer than convenient.
But actually picking 1-2 vassals that are highly useful to you, could make for more interesting opportunities where you are eventually having to either lose that customized vassal or have a mostly ineffective diplo slot.