Opening up the borders currently when the world is fighting a pandemic like Corona is pure stupidity which is why I don't agree with your idea at all op.
This should only be considered when the vaccine is administered on a global scale and majority of the population needs to have access to it. Currently, the best way to strengthen the US economy is reducing taxes in my opinion.
If there is a proper measure, I don't think opening of the border would really be a big problem and threat to a country relaxing and open of borders in full operation. I believe there are machines that cna detect easily and individual that can detect someone when they come in contact with such machines. Ones one is positive, it will detect it.
Talking about vaccine, no one is willing to take it, no one is ready to take the risk of been test with unknown chemicals (literally to a layman).
There is more to go about coronavirus eradication, I still think we will be face with more challenges as we jump into. A new year, we can only hope it comes with ease.