We have already waited long enough to migrate our forums. Now that xF 1.1 series is out, we have only barrier of importer, stopping us from migrating our forums. We would have chosen vB 4.0 -> xF 1.0 -> xF 1.1 path but we don't want to lose handful of thread prefixes (we are ready to sacrifice with tags though). An alternate route I can think of is to first downgrade our vB installation from 4.0 to 3.8 and then migrate from there to xF 1.1. But unfortunately, I could not find a good way to downgrade it. Any pointers will be helpful if it is possible at all.
Another option is to modify the vB3 importer. The schema for vB4 is virtually identical to vB3 except for a few areas such as attachments. If you aren't up for the task then you can try to hire a programmer or post a request: