You should always be careful, even when using a contraceptive. Vitamins shouldn't decrease the effectiveness of the shot, from what I know. Usually it's only anti-biotics that can do that. Though do start being careful if you are not ready to have a child.
is it really necessary that i use condoms since im taking the shot? my doctor says its not very likely i would get pregnant while on the shot that it could actually take up to 2 years for me to get pregnant after i stop taking it