Turning Durkon into a Vampire most likely won't make him evil immediately, if only because he would be under Malack's control - he's not making choices, so why would his alignment change? If Malack were to be killed, however, he's left to make his own choices. Even then it would depend on how he handles his affliction. If he became evil he would lose most of his priest class, and would be as good as dead plot-wise..

Being killed by a fireball from V or whatever other intervention will still happen is more likely IMO, after which the Order would high-tail it to the Dwarven lands to get him raised and prepare the last gate. There's not much you can do against a lich without a functional cleric/healer, anyway.