After you fill your vial until it doesn't take any more of your blood go back to the caravan
That is what I did, and nothing happened. I just died from thirst. I don't think I missed anything.
There is a issue in the game now, if the vial looks full.. it's not, so you need to give it extra blood. and the caravan area detection is big so you will have to fill the vial away from it
