Vitamins in so-called "non-active" form are unlikely to make a huge difference, because most people have no or a small deficiency or, if there is a higher deficiency, not for long.

However, the "active" (in case of folate or B12, methylated) form can help in case of insufficient enzyme activity which doesn't just go away (if genes or some chronic biochemical abnormality are at play) because the (small) problems continuously build up over time until they are bigger problems.

Of course longer episodes of mania or depression can be different, since often your appetite is virtually non-existent. But continuous intake of vitamins when you're not manic or depressed is mostly unnecessary. Besides, eating or drinking dairy products, meat or eggs might give you the end-product of vitamin-mediated biochemical reactions without getting "enough" vitamins. If you are vegan, you might have more of a problem.

There is one big exception when it comes to problems with deficiency and that is vitamin D when you are depressed and don't leave the house much. So I would suggest taking that one. When manic, however, it does probably more harm than good.