Oh, there's something odd going on all right! Actually I could have spun that out for about another ten chapters if I'd felt like it. =:P The clues have been there as we've gone along, but (I hope) not so obviously that readers will have worked out before getting to the final chapter why Vernal is the way he is.

Vervain doesn't appear much at all in the film -- his biggest line is near the end: "They had a great bird that turned into a shaft of lightning!" -- and yes, he is indeed voiced by the one and only Derek Griffiths. But in the book he seems to me to be probably the nastiest rabbit in Efrafa, since at least Woundwort genuinely believed he was protecting his people. The last bit of the book implies that Vervain got killed by the elil after the final defeat, but who knows...?

Thank you again for the comments!

That Vervain line is quite near the end -- it's about 2:15 into part 9. =:)