So what happens when you get the call if you are on vacation? Don't you have a limited amount of time to get to the hospital. You can't say I will be there the next day can you?
I'd say for international travel you basically probably need to give up the offer, but be happy if you were a primary for a call. If you are in another state and can drive or fly in say 6 hours it probably would be possible but you run the risk of rushing back and then them not transplanting you for whatever reason.
Don't delay your life waiting for a transplant. I had three and half years were it looked like a transplant was close, in that time I visited (and received dialysis treatments) Singapore, South Africa (twice), Chile, Argentina, Brussels, Montreal, Ireland, Spain, Portugal and took internal US trips. All of those vacations added to my quality of life and I'm VERY happy I didn't stay around home because it felt like a transplant was very close.
I vacationed extensively well on the list. Its a nice thought to think you might be able to get back on an overnight trip but not really practical. I'm in DC, my first call was when I was on vacation in Cabo San Lucas Mexico over a Thanksgiving, they were calling fairly far down the list so I might have had time but it was not a good match.
The only time I flat out missed call was the Brussels trip where overnight 7 hours later I found the message where they wanted someone that could have came into the hospital quickly - so that missed call wouldn't have worked because I would have had to wait for say a 10 am flight back home that would take 7 hours and they wanted to do the transplant long before that.
My real call resulting in a transplant was at 5 am with them telling me to come into the hospital once they got the room ready. I had time to take a bath and use public transportation to go in (versus rushing in with the car and paying for parking). I don't think I was actually sent down to be prepared until about 2 pm. But they were pumping me with IV medicines starting after I arrived.
Otherwise I received a ton of alternate calls and for those it took well over six hours to figure out if I was even a valid alternate.