Whenever couples take a vacation together who normally pays for the trip? Does the guy normally pay for everything or do they split cost ?

Married couples with combined finances will usually save up together or go into joint debt together to pay for a nice vacation. Or if they keep their finances separate they'll each contribute half. There are no rules on this other than that for unmarrieds neither party should spend tons of money on the other. They should go dutch and if the budget is too high for one of them they'll need to make it a cheaper trip. 

If they are a bona fide couple this issue arises everywhere all the time.  How much money each earns is relevant.  Here we have a lot of farmers and their money is committed to the farm.  So the wife goes out to earn some outside money and uses THAT to provide the holiday.

My husband and I deposit our paychecks into a joint account.  This account pays for vacations.  It also pays for bills, groceries and other joint expenses.

I’ve been married over twenty years, and there has never been a question about who is paying for a vacation. If we want to take a vacation, we agree together where we want to go, what we want to do, and calculate how much we will need to make that happen. Then we save together to meet that goal. 

Same way you handle your dating expenses.  Some couples are perfectly happy with the male paying for everything.  Some couples take turns.  Some couples, the one who suggests it pays for it.  For me and my wife, dating started with taking turns paying.  I paid for some, she paid for some.  Once we moved in together we divided responsibilities.  For our vacations, I make more money so I pay more expenses. I'll buy the travel and cruise tickets or whatever.  If we want to do something extra like disneyland tickets she'll pay, or buy dinners, souvenirs. 

How do you do it?  You talk about it.  Might he get mad if he's paying for everything and you're not helping at all? Maybe.  It doesn't have to be 50/50.  But there usually has to be an effort made by both sides.

If we go away on a vacation, I pay the full cost. We are not married, and I asked to take the vacation, I made the offer, why would I expect to not pay. We both have a good time, I love sharing.

My husband and I don't think like "this is my money, and that's your money". We have OUR money to take care of OUR family. So WE pay for vacations. It was like that from about 1 year into our relationship until now, after 10 years and marriage.