I suppose it depends on how vampires are run in D+D, and in OOTS in particular. In a lot of vampire stories, becoming a vampire isn't character growth, because the character is dead. The vampire is a different character who looks the same as the old one, and perhaps has some of their memories, under the control of someone else. It is character development of a rather forced and non-consensual sort, in the same was as casting a permanent Dominate Person-type spell on Haley and have her not care about treasure would be.
But in response to the OP's point; this would indeed be a good way for Durkon to fulfil the two prophesies. The next question would be: would he be returning to the Dwarvern homelands as a member of the Order, or of the Linear Guild, or Team Evil, or as a lone avatar of doom? I suppose the answer depends on how much of the ‘real’ Durkon’s personality is left in the vamp Durkon, and how much control Malack (should he survive this arc, which I doubt) could exert on him.