I stayed with vb3 for the same reason many sites have. It was fast and just works. However, it is also old and dated. I watched friends go through the vb4 (or vb3.9 as I refer to it) nightmare and wanted no part of it. I bought an IPB license because it was not an IB product and it looked clean and solid, only to dive into the code and learn how bloated and un-optimised it is. 90+ database queries on a single page? Who does that? I cannot imagine running a high traffic site on IPB without a cluster of servers.
I used the built-in VB3-to-XenForo importer to import all of the data except threads and posts. Based on a sample test conversion for posts and threads, I estimated it was going to take 24+ hours to convert with the built-in importer. So I wrote a custom cli-based program to convert the posts and threads, but it was only slightly faster than the built-in system and was still going to require way too much downtime.
So instead I decided to try converting the tables inside mysql. The mysql conversion method took about 30 minutes to convert the 5+ million posts on my development server and I expect it will be even faster on the live servers. So far, everything seems to have converted fine though I still need to spend some time looking more closely at the post/thread tables to make sure everything converted correctly.
Well that's my story so far. I look forward to becoming an active member here and if anyone has any additional conversion tips they can pass on, that would be great! I hope to do the live conversion in a week or so once I have had time to create a custom theme.