We recently had a client who needed to convert their WordPress website to Joomla. Their forum was vBulletin and they used aMember to manager their users. During our initial research, we discovered there was not a direct migration path to Kunena, however, we found that migration was still possible if we converted vBulletin to phpBB3, then to FireBoard (a predecessor to Kunena). Once we had a Joomla 1.5 website set up with FireBoard and a successfully migrated forum, we upgraded to Kunena 1.6.x. (We hope to add basic instructions as to what we did at a later time)
We ended up having to modify the phpBB3 converter script so that the aMember user data was imported into JomSocial (the replacement of aMember) along with making changes to the converter for phpBB3 to FireBoard. All in all, we were very happy with the results.