I take sublingual vitamin B-12 (Walmart sells it, as well as other stores) and haven't noticed any weight loss. I do think that when you diet, you benefit from taking a multivitamin because you start 'dropping' foods from your menus.
What causes you to lose weight is eating fewer calories. If you give up some nutrients along the way, that's where a multivitamin will fill in the gaps. That's just 'insurance.'