Instead of making use of Memcache specific code why don't you simply make use of vBulletin cache, and if your vB uses xCache/Memcache/etc., then the hack will get advantage of this? Also if you go in this way you will not need two versions of your mod anymore as because if your vB doesn't make use of any cache then nothing will be cached.
Take in account that caching in memory is faster than storing the information on the database and both ways are stable.
My two cents