Did you just copy the postbit_legacy template to postbit? if you did that might be a problem or if you edited in anyway. If you did set the setting in vBulletin Options to use the postbit_legacy over the regular postbit you did it right, but did you edit the postbit_legacy template at all? If you did you might have messed it up and the best option is to revert it back to default and try modifying it again. If you have any other problems with this feel free to post again

and we'll do our best to help you.