As far as I know a vampire doesn't randomly give the blood kiss to near dead people, it'd most likely just feast on its blood, I can't think of a reason why it would make an exception to this random nearly dead man.
In a similar manner, the soldier would have to be sigmar to impress a vampire into giving him the blood kiss, because why would a vampire that got close to beaten want to augment the power of his enemy? That and I wouldn't think that a vampire would give the blood kiss like a reward, invariably they do it for more selfish reasons.
I therefore propose my random fluff background: The vampire is off into the chaos wastes because they are seeking a chaos artefact (absolutely not malus darkblade

) or whatever, that is up to you, plenty of reasons for a vamp to go to the north. On departure he finds a strapping young local boy and vampirifies him so that he has an assistant for the expedition that will not need direct commanding like his usual thralls, also the increased bloodlust of being a new vampire will mean that your character will be very efficient at finding food and killing it for the elder vampire, so would be useful for him on the journey. Seeing as turning into a vampire is a very traumatic experience, your character will be particularly loathful of chaos as he had to spend so much time in its presence up north. This would probably also explain how he is so suscetible to it as well.
That seem okay?