Are there any drawbacks to having a VBR at production that only handles backups, and one offsite that only handles replication?
The only drawback I can think about is the fact that both servers are unaware of each other.
Another approach, would it be ok to have the VBR that handles backups and replication located at the target site, with proxies and repos for backups at the production site?
Correct, this approach is among recommended ones.
Could I suggest a feature request of redundant VBR server, both onsite and offsite, that are synced with one another?
Your voice has been counted.
And at it's core, it's really just 2 VBR's that are synced with one another in real time.
Tenants' replication specific information is stored directly in SP's database, so, it's still a functionality of one backup server or in this case Enterprise Manager that can be made available to outside world via RESTful API.