Taking a multivitamin is a great benefit to health. You don't need the high priced ones either. For men, you'll want to get one with Iron in it.
Actually, you will want the opposite. Most men don't need extra iron, women generally do based on losing iron in menstrual cycles.
That being said, I've been vegan for a while so I take a few things just to keep me healthy. In all honest, I don't need to take them. I am healthier than I ever was when I was an omnivore, but it's just an easy way to stay ontop of things.
Multi-vitamin - I just take a cheapish multi-vitamin. I don't really need it given that most of what I eat is fresh fruit and veges, but they are cheap and it doesn't hurt.
Iron - I take a small iron tablet a day. Again, not really required and in some men, it can be too much. I need to take a bit more iron than most as I don't have a source of heme iron.
Vitamin C - Mostly during winter. But I also need it to better absorb non-heme iron both from veges + the iron tablet.
B12 - The one vitamin that vegans can't get. Something like up to 80% of vegans have a b12 deficiency. There is NO source of natural B12 for vegans, so this is important for me.
I also recently tried taking magnesium at night which acts as a muscle relaxant. I do BJJ so my muscles can be incredibly tight after training. In truth though, the level of magnesium in the pills is pretty damn small. You would probably get more from eating a good steak (Of which I can't do so

).
Alot of people hating on supplements in this thread. In some was they are correct, if you already have a good level of a particular vitamin, then plowing your body with more won't do any good. But for most people, supplements are just making sure they are reaching that level. And for people like me, it's putting something in my body that I am unable to get elsewhere.