Oh, yeah, there's definitely issues with Woundwort and Vervain as a relationship on both sides. Woundwort is, well, Woundwort, he's a control freak to say the least, and I love Vervain, I love how delightfully cruel and conniving he is, but I don't think that he could foster any kind of meaningful relationship given his longstanding attitude. Vervain's never really had any kind of pathos or sympathy attached to him, nor does he seem to see any value in any genuine relationship.
If anything, if I were writing a fic with the two of them as the primary relationship, I'd completely erase most of season three, do away with the whole destiny thing, and have the two meet again under circumstances where Woundwort is having a journey of introspection, which he wraps Vervain up in too. The two come out better for it, having learned a lot about each other and themselves.
Come to think of it, the two are mirrors and polar opposites in terms of character. Woundwort is almost impossibly large, strong, and commanding. Vervain is almost absurdly cowardly, pretty scrawny, yet he shares (almost) all the cunning of El-ahrairah. They both suffer fatal flaws, ie, severe cruelty. It's an interesting thing to ponder, or, I think it is at least.
I need to seek out that "come to my burrow" fic...