In the morning would definitely make more sense and be more useful for you. This is because when the vitamin starts to take affect, if you took it in the morning you will be doing your daily activities when it takes affect. If you took it before bed when it took affect you would simply just be sleeping.
Depends upon the supplement. Most vitamins (a b's, c, d, e) and minerals (calcium, magnesium, etc) need food to metabolize properly. So you'd want to take them with meals. There are other supplements for different issues that are better taken at night, such as red yeast rice. If you are talking about a simple multivitamin, best to take it with breakfast.