I don't think a corporate policy can be made mandatory on the board unless they expressly agree to be bound by it (effectively making it a board-level policy). A corporate policy requiring vaccination to enter the premises might be enforceable but I'm not sure it could be used to deprive a director of their right to attend and participate in a meeting.
So we were ready to provide accommodation (virtual option) in case one or more directors were unable or unwilling to vaccinate.
But like others on this thread we dodged the issue because our board voluntarily chose to follow the corporate policy insofar as meeting attendance is concerned (but other aspects of employee policy won't apply to them, like consent to disclose vaccination status, since it's not relevant to directors).
It also became a bit of a moot issue for us because our next in-person meetings are off-site and the venue is has mandatory proof-of-vaccination requirements for attendees (as dictated by provincial and municipal medical officers of health requirements).
I agree with other posters that it's great if the board can lead by example and agree to abide by the corporation's vaccination policy but based on statistical vaccination rates I think you should be ready to accommodate one or more directors who are unable or unwilling to do so.