Simple question. Is there anything that controls which character hostiles will go for in combat, and if there is, how does it work?

I'm getting quite tired of everything going for Virgil when Sogg Mead-Mug is standing right next to him, ready and willing to tank.

I stuck a ranged weapon on him and he likes to use it before swapping to a staff (how can we miss at point blank range, so annoying). He seems to be fairly good at tanking damage in my two games though.

He has no Bow skill though. And he will still stand in melee range with his bow.
I wouldn't say that he's good at tanking. He's good at getting aggro. But getting aggro is not tanking. Being able to withstand damage is tanking. And someone like Sogg Mead Mug, or Gar, to use early-game examples, are much better at that.

Doesn't matter if he has skill in the ranged attribute, it matters that he lets others run in first.

His ability to tank comes from his healing... works for the start of a game.

Most fights don't have a "run in" element to them though. You get close to one monster - you get rushed by a whole group of them.

And healing really adds nothing to tanking ability. Tanking has to do with how much damage he takes, and how much damage he can take before he dies. And in both those areas Sogg Mead Mug is considerably better.
In other words, you'd be better off with a creature hitting Sogg, and Virgil healing him than with a creature hitting Virgil and him healing himself.
This is basically why Virgil's tendency to get aggro, and his completely retarded combat behaviour is such an annoyance.

Can always tell Virgil to back off. And Sogg to initiate combat.