CBC News on US-Canada Border Plans
The article basically states there is no plan, but border state US politicians and the auto sector are pushing for it. US Rep. Brian Higgins (NY D-Buffalo & Niagara Falls) would like to see it open by July 4th (under certain health-related restrictions - "you've been vaccinated, certify that you're going to wear a mask, certify that you're going to practice good physical distancing."
The problem I see with his restrictions is that they involve active participation of people. Maybe I've just gotten too negative about people, but I think people will "certify" anything as long as they'll get to do what they want. And as others have pointed out, proof of vaccination will need to have some sort of government issued documentation. The COIVD vaccine card I have in my wallet is way too easy to fake. I'd guess the bureaucracy for a certification program will take a while to form up on both sides of the border.