Usually when you wake up or after a workout to replenish what you ahve lost. If you aren't lacking any of the nutrients then you will just excrete it. So at night (usually) your diet has given you everything you need. Some will tell you to take two or more a day, so this only applies if it is a one-a-day vitamin

Take it with food, as for timing, it really won't matter, your body is pretty smart at using what you give it.

With food.

with post work out meal

It doesnt really matter IMO..

There are lots of differernt vitamins but for e.g. lets take vitamin C which helps with repairing of tissue.

After a workout your muscles need repairing and some people would suggest taking vitamin C immediately after a workout but the reason this is not necessary is because your body will store the vitamins/minerals you eat quite efficiently regardless of the time you ate them and use them when required.