If you'd like Free, it think it would be appropriate for Alaira to suggest the counter ambush. If she's worked in Djarkel long enough she'd know enough about vampires to know that they would be strongest at night, and unless the group wastes a few more days trying to avoid them they're going to end up fighting them one way or another. Vampires don't tire out as easily as they would and could cover a lot of area. However with the knowledge that you guys got from the guard captain and the map, you can more or less figure out where their base of operation is, and they wouldn't expect and ambush on them. Alaira could point out that since they're going to end up fighting anyways, they might as well take the advantage and strike first instead of getting caught off guard. Not only that but if vampires are responsible for the attacks recently than it would be even better to attack them at their base so you can collect their intel. As for the caravan master, even Alaira could deal with him: Intimidate check to make him listen to her and as long as Alaira makes her argument sound mostly logical, he'll begrudgingly accept it. This final fight will be inevitable unless you all act like complete cowards, so it falls between the choice of continue the path and dealing with them at night, or going on your way or taking the fight to the vampires and putting an end to their actions once and for all.