Retin-a should be applied at night and not in the mornings. There is definitely a warning of photosensitivity with retin-a, however, with newer formulations of retin-a this is usually not a big problem if one is using sunscreen. Furthermore, retin-a inhibits many enzymes that are stimulated by the sun to break down collagen and elastic fibers (matrix metalloproteinases) that cause photoaging, thus using retin-a if you are not suffering from the photosensitivity is an important tool against the deleterious effect of the sun.
Just use sunscreens with physical blockers such as titanium dioxide or zinc oxide and you will be fine.
Have fun on your vacation