This one is interesting because Vault has an enterprise product which I assume (hope?) Fly is paying for. That enterprise version includes performance replicas,

This one is interesting because Vault has an enterprise product which I assume (hope?) Fly is paying for. That enterprise version includes performance replicas, which allows for cross-region replication of secrets with region-local reads (and slightly lower writes). The OP almost makes it sound like they are using the non-enterprise versions (or at the very least, not taking advantage of this particular functionality). That said, I'd imagine with large enough scale, these sorts of features break anyways.