converting to a fresh and modern piece of software will also mean you adopt some of its principles, and let some of the old things go, or do them in the new, modern way instead of trying to replicate exactly what you had in vbulletin
I think this right here is really important. You're looking for vBulletin to be duplicated in another platform. That's not XenForo, and this problem crops up a lot in the IT world.
Many times, I've proposed a change to our corporate software and too-often the answer is, "okay, existing software does <list of a hundred specific things>. Does new software do that exact list?" Well, no. It's missing some things. But there are these other things that it does a whole lot better, and a few new things that the old stuff doesn't do, and I think it's worth the tradeoff.
If you want to wholesale-duplicate an existing list of functions, you're probably never going to be happy. Different software designers have different philosophies and it's unlikely that they'll both settle on the same solution. This isn't VB on a different code base. It's different. You'll have to change some things. You might lose some things. The question is, do XF's strengths outweigh those for your purposes? Only you can answer that, and maybe your existing answer (no, we need all those things) is the right one. But it doesn't appear that you've considered that you maybe can't retain all backwards functionality forever, or that you could change up how you're doing things. So just consider it.
Google Analytics might be helpful for this. Sometimes I grab onto things and I think they're absolutely critical... and then we check the analytics and it turns out only 5% of the community use them.
Anyway... just some thoughts from someone who is converting a sizable board over and is accepting that a few things will get lost, a few things will be gained, and hopefully we'll be a better community at the end.