As long as it's their own money, so what? It's their choice. Using their money to go on vacation and using some of their vacation time as a witness.
Though i'm not so sure about groups of women taking off on Missions leaving their family behind. That is the kind of behavior that is supposed to be restricted when married, unless done together. Which is why Paul was not a fan of marriage. You either have no limit your availability to God or not be there for your family as you should. But, in America, we have to have it all, why let our commitments to others limit us to doing what WE want? =P