I had high hopes, but it doesn't work. I tried to go from Vanilla v2.1.x to phpBB v.3.0.12, as well as directly to phpBB v3.1.1 (with some modifications to the converter script), but the result is all messed up.
During the run of the script something with script execution seems to always time out, causing the before mentioned generic 500 Internal Server Error, which btw does not have to do with the PHP max_execution_time (as that one is set to 30 sec on our server and the server error comes a lot later). That server error does not seem to be crucial, because with the help of the generated cache files in /convert the script seem to continue on restart. I had to restart the script quite a number of times for our database with about 75 MB to finally get to the result page, but that one showed a long list of CatMap Errors
and some PrivMsg Address Errors
at the end.
It seems that all Vanilla categories have been correctly transformed to phpBB forums even with correct forum description, but the numbers are all wrong (seems the number of posts has been also inserted in the number of topics column, both show the same value now), last post to forums set to board admin at conversion time, topics in forums are missing postings and show "Guest" as OP and last post by with zero replies, opening a topic show the proper username(s) but most of the time only the initial posting with the rest of the discussion missing.
A conversion in this state is totally unusable, but I'm not the right person to start tinkering with the code. It looks like a large part of the conversion would work, but some errors prevent a usable outcome.
If only someone with the right knowledge could look it over and adapt/correct this conversion script to properly do a conversion from Vanilla v2.1.x directly to phpBB v.3.1.1 it would be so great. Right now I'm stuck with no idea what to do.