you can't force people to play something they don't want, they'll simply leave and play something else.
the only thing that -might- balance things out is making rewards drastically lower/higher depending on the cop-criminal ratio. So if there's 1 cop online bank robberies, vigilante bonus and everything criminal related (criminal, not civilian) shouldn't give you more than like 1k - the same you get for a few minutes of trucking since both of these are risk free events in that certain period of time. This might make people switch over to cops since cops would get a huge bonus on arresting/killing criminals. Imagine getting 10k for a single arrest cos you're the only cop in town. Might actually be a decent way to lowkey force people switch sides..not really force but more like bribe them to switch. If its been suggested already just ignore it, i haven't really checked.
You might even include making ammunations cheaper to those 3 cops/civilians on the server, and more expensive to the other side. Again this would take a lot of testing and tweaking to actually balance things out but it might be worth it.