I think the underlying problem is that there is just not any real cooperation on global vaccine distribution. Certainly the details about the enforceability of the various contracts wouldn't matter if the customers all talk between themselves and agree on what would be acceptable modifications of the delivery schedule.

In the event, it seems neither Boris nor Biden seem very interested in considering anyone else before they've scored enough points at home, and meanwhile real international cooperation on vaccine deliveries is dead. Oh well, hope not all trade follows that pattern shrug.