Divide and conquer!
Players of that skill are probably proud and don't like people stealing their frags. So they normally want to finish their "own" fights.
Lure the vampire away from the troll by having a npc (closer to the vampire than to the troll) appear and shoot at him from not too far away(so he will still decide to attack that one in close combat) right after the vampire finished off his last one( that way you will kinda force him too attack the enemy you picked for him). After attacking that one lure him even farther away, same game. then have him attacked from two direction. there should always be one in his back. Make them aim for the head too (no regen), wear down his armor(the vampire might regenerate, his armor doesn't), and finally him. Those npcs don't need to be skilled, they just need the right tactics.
With vampire out of the way, let someone sneak to and surprise attack the troll in close combat, if its his weakness. Called attacks with +4 dv are important to go through his armor.
Used acid spells early on him. He might laugh while soaking the damage. Then tell him his armor rating has halved.
For shooting use cover, thermal smokes. The goal of defending npcs is to delay until they get stronger backup.