Check your vBulletin Options and ensure the default blocks are set to valid block IDs (block IDs can be found in the Block Manager).
If that is set and you still experience the issues, try flushing any cache systems you may be running like vB Optimise or vBSEO.
3rd and final thing I can think of would be to edit and save any of the blocks in the Block Manager to see if that rebuilds the cache.