Vitamin D is very vital for all sorts of things - immune system, bone density, fatigue, aches and and pains all improve when our Vitamin D levels are right. I also think that when we are on holiday we are more relaxed so everything will seem better.
As CKLD has rightly pointed out - what we wear can have a big influence! On holiday in a hot climate we tend to wear loose clothing and possibly swim in the sea and these things might well help the situation. I do think stress has a big influence on our ‘lady bits' - if we are busy and stressed then we are more likely to be ‘clenching' down below. If you have a job that involves sitting at a desk then this will certainly not help atrophy.
I doubt that vitamin D alone would solve urogenital atrophy. it's simply one aspect of a whole range of things that will help improve things generally. DG x