I don’t know how the importer works exactly, but shouldn’t it be doable in one run? When a quote references a post, that post has to be older than the one containing the quote, so the importer should already know its new ID.
Either way, it’s an important feature, I think.
Not really. There is xf_import_log table that stores IDs map (until new import is initiated). So you need to only go through all the posts and replace old IDs with new ones without needing to do something with vBulletin database anymore.