They have not pupated yet so you should leave your vines until they are gone. They will crawl away and form a chrysalis before they become a butterfly. By composting the vines you are most likely killing the caterpillars. Once the cats are gone then go ahead and cut your vines back but I'd look carefully to make sure there's no chrysalises on the dead vines. Sherry who occasionally posts on this forum will be able to give you info about the Gulf Fritillary caterpillars/butterflies. Hopefully she'll see your post.

Ditto to what Karen said, plus if you've got small, not-ready-to-pupate caterpillars on your vines, they won't crawl away, unless they've completely skeletonized your vines - at that point, they might crawl off the vines looking for more passionvine leaves. I would leave the vines as they are all winter and not prune them back until spring. You MAY find that some caterpillars have survived the winter - they've done it here - and in that case, just cut off the section of vine they're on, prune back the vines, and lay the caterpillars at the base of the vines so they can eat new growth as it pops up.
Thank you for caring about the gulf frits!
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Sherry

Thanks Karyn and Sherry. I'll go ahead and leave the vines alone until I see new growth in the spring. I do want to clean the area up a bit. As it is now, the vines are climbing old nasty blackberry canes and other weeds. Would like to get next year's growth up off that and more onto the fence or maybe make a trellis or something. And I'm going to plant some lantana nearby for the butterflies.

Thanks for all your suggestions. I think I've got yet another new interest -- my wife will be oh, so glad.