Some people are way too harsh on others and act like they're perfect. We all make mistakes and we all commit sins (mortal or venial) of one type or another. If anyone here can say they never sin then all I can say is "My Lord and My God!" I mean seriously.
The fact is, yes it's a mortal sin. You missed Mass without an extreme circumstance causing you to miss it. Now go to Confession, do penance, and next time try to plan your trip in a way that you don't do it (whether that's going on Saturday night or finding an earlier mass to go to, etc.).
However, let's not forget that other things should not come before God no matter what. I can understand if you're somewhere that there are no Catholic churches anywhere nearby, but otherwise the effort should be made. I know it's hard when you're on vacation, but to me that's a time when you really show what's important to you. Will you go out of your way to go to Mass? When you're not following your normal daily/weekly routine? That's almost like a test of faith, no?