Hi Pauline...
I did our switch gradually too, mostly because my dd had been allergic, so we wanted to try the switch slowly to look for problems. But I think I would have done it that way regardless, just because it's so new to them, you know? I'd also stick to one "big change" at a time, I think that's important. So if it were me, I'd replace an ounce of formula with an ounce of milk, (so if it was a 6 ounce bottle, there'd be 5 ounces of formula to one ounce of milk) then wait a week, then go to 2oz milk/ 3 oz formula and so on. If she won't take that first bottle, then maybe back the milk down to 1/2 ounce and go from there.
My dd is 19 months, and she still has her morning bottle and her bedtime bottle. This is totally because I am not willing to give up that time with her just yet.
She takes her cup no problem for snacks and at mealtime, but I need that cuddle-time still!! She wouldn't miss the bottle at all, I'm sure, she was never very attactched to it. I'm the one who'd be crying!!!!
We do the milk first thing when she wakes (I have not given a thought to stopping that...
?) and b'fast about 30-45 minutes later.
And yes, I'd enjoy your vacation and tackle this once you're settled back at home and in your routine.
HTH!!