There will always be something - a holiday, a vacation, a birthday, visitors, plans, life is always happening. When you're in the middle of it, there really is no time that says "HEY! Next Tuesday is a great day to leave/confront him/lay down some boundaries." But in hindsight, it turns out that Tuesday really *was* a good day for that.
Laying down our boundaries doesn't always mean you have to confront your addict. In fact I sometimes find it's better when I don't. Because sometimes when I think I'm setting boundaries, I'm really looking for a fight in a self-righteous way. What are your boundaries? Do you know what they are? Even if you haven't enforced them yet, do you know what needs to happen in order to be happy?
Finally, you are the parent. You are supposed to do what's best for them, and material comfort does not always mean best. If you feel okay about going on vacation then go - but I would think strongly about how you will feel & act while you're there. Can you detach from his situation enough to enjoy yourself? Because if not, I see no sense in going.
Nothing changes if nothing changes. It seems like you were making some real positive changes, but then you stopped. Why? And does it surprise you that once you stopped changing, he did too?