A member of my husband’s family didn’t speak to her son for almost 20 years after he eloped while she was on vacation. That was in 1970s and it still has ramifications in the family today.
Be very careful. Even if you think that your families will be ok with it, they may surprise you, and not in a good way. At best, I think it would be polite to give your parents time to get used to the idea, rather than telling them on vacation. If their initial reaction is not 100% happy, and you are all stuck together on vacation for several days, there could be a lot of hurt feelings, and damage done to your relationship with them, that could take a very long time to repair.
Good luck.