Taking Vits now wont make any difference as it takes a given amount of time for them to get through and into your system, pre op your wasting your time unless your told you need to take something by your GP, some like to start early as it gets them in to the habbit of drug taking early, (it wont make a lot of difference to your health as the obesity will outweigh any good done by a multi vit) i never took anything and only started vitimins about 4 week post my op.
As the others have suggested, have your blood test and see what the results turn up !