I should say a couple of things need to happen here:
a) he needs to recognise he has a problem with drink (not the same as being an alcoholic). Sitting alone in the pub and drinking vodka whilst your resentment starts simmering is not a healthy way to behave around alcohol; it is not the same as getting merry at a party. The problem may not be that he is an addict or that he is drinking enough to damage his health; the problem is that he uses drink in a problematic way iyswim. which leads onto
b) his resentment. If he has a residue of resentment from his early life he needs to find a way of treating or dealing with this that does not affect his children. Remember they are going to be teenagers and teenagers require you to be strong and resilient in all sorts of ways. It would be wise to deal with any issues now.
c) setting boundaries. From what you tell me, it sounds as if he tries terribly hard to be a wonderful dad until he starts resenting it and feeling sorry for himself. He needs to find a way of being just wonderful enough not to resent it: to be able to say cheerfully "that's enough, I need a break now". There shouldn't be a reason why an ordinary dad on an ordinary holiday should feel so miserable and resentful that he has to start downing vodkas- presumably you cope without?
d) his relationship to you. I know you said you have been ever so much better, but how much of that is actually about you recognising your problems and sorting them rather than the two of you sorting out your problems? The impression I get is that you are taking on the responsibility for this family's happiness, and that is never going to work. If you are to be together it has to be joint work.
Not saying LTB, just that these are issues that need to be worked through.