From what we've seen, and the old priest has said, my guess is the Vampires are scared of Dhampir for some reason. Maybe the titan priest's whole "return of the first vampire story coming true".These vampires really got nothing better to do, making such a big fuss over just one somewhat useful errand servant watering down his bloodline.
They're all immortal, they should just take note of it, then ignore it for a couple ten thousand years until the Dhampir dies of old age, and be powerful enough to win even if the Dhampir trains for a million years.
If they can't do even that much, they shouldn't call themselves immortals.