He's a significant threat to people, who just so happens to now be at his most vulnerable, so they let him go because "escort job first". Both reason and means for his death are present, but the plot just says no and that's that. Textbook plot armor.
He's not at his most vulnerable yet, the sun isn't out yet. The vampires objective is not here so there is no point in him wasting time and taking risks, hence he backs off.
Rama was told off which makes sense since chasing the vampire means potentially putting the people they're escorting at risk. Not that Rama can chase after, or guarantee that she can beat the vampire anyways.
I don't know what plot armor you speak of lmfao.