I would have thought in the case of International Borders, all inbound passengers would at least be required to test negative assuming a reliable quick test is available by then and quite probably vaccinated as well. For outbound passengers almost certainly vaccinated, but I wouldn't think forced, just refused if you are not. It would be foolish to want to travel without having been vaccinated. It will be interesting to see what actually comes about. Surely most countries would require vaccinated prior to entry. It'll be a couple of years before we find out.