Thanks for posting this question. I've been talking with my wife about the same thing. While it's all speculation at this point, it's still nice to be able to bounces best guesses off of each other here.
We're hoping to make up our Europe trip that was cancelled this summer in July. Since the health experts, at least here in the U.S., are expecting the general public to be getting the vaccine in late March, into April, I'm hopeful, but I'm trying stay grounded.
To answer your question, I think that we'll likely see a proof of vaccination requirement. Under that assumption, I am hoping that we can get appropriate documentation when we get our shots, so that whatever the proof requirements are we can meet them without jumping through too many hoops.
At least we are starting to see a trickle of real information coming in now, such as Pfizer's effectiveness and expected roll out, along with health experts providing a tentative timeframe for general public roll out. Next couple of months should help fill in the blanks and give us a much better picture of what and when for international travel.