I would prefer to believe that vampiric hypnotism can't force someone to do something entirely against their nature, only sway them to a different choice they might have made anyway (rather like the multiverse discussion in Ted Chiang's "Anxiety Is the Dizziness of Freedom"). However, as you say, there do seem to be some cases where people on FK were hypnotized into doing things that seem likely for them, like reporters giving up an investigation. Perhaps in some of those cases the fear of encountering a vampire and knowing they'd die if they continued, combined with hypnotism, provided sufficient incentive? Hypnotism might almost become an excuse to give in to fear in that situation.

(On a tangential note, the North American version of Being Human had vampiric hypnotism, but got around it being the easy solution to everything by making the main vampire character absolutely terrible at it. Since he rarely and only reluctantly dealt with other vampires, he had to seek other solutions most of the time.)