I'm in the same boat you are. I had to have a custom conversion done on a DNN forum, we didn't even know how it was going to turn out. It worked, but there are other parts that were not included in the conversion.
So while everyone hashes out what addons we should use and how we are going to duplicate the non-converted items, it becomes a process of trial and error and you would be left with possibly a bunch of code hanging that is not needed because you took a different route with functionality duplication, etc.
What I would do is:
- perform an initial conversion on a copy of the live database
- disable e-mail notifications (so that your live-site users don't get alerts from the new site)
- set up a new category for various forums, thread, etc for discussing how to go about making all of the functionality of the VB site work on the new site
- Document your steps along the way so that you can duplicate things for the final conversion and configure things with minimal downtime
- Shut down the live site and perform the final conversion to a clean-slate XF install, install your addons, bring in your custom items, configure everything, etc
I suppose that you could bypass the need to rebuild everything. In this case, you would still do 1 & 2 but before you import you would delete all of the lite-site categories from the dry-run site, and then nothing would be duplicated (and you would keep your internal conversion discussions in tact). But the problem is that you might do all your tweaks and modifications, and meanwhile, posts are still being made on the live site and if there are changes made that involve the functionality you need to re-create, those new changes will not import (since you basically had to re-create them manually during the dry run)
I just think it's cleaner to document all the steps you need to take to duplicate lost functionality, then start from a clean slate. That way you database isn't potentially bloated with tables and field full of unused addon content, posts, private messages, etc. But that's me, I'm a clean-slate kind of guy I guess