you have hit the issue in the head. Even other tools like victorOps suffer from this. Overrides are manual and in some cases, if people switch 4 day on call with 3 day ones. They are getting screwed. There has to be a concept of vacation and automate rescheduling. I am suprised, there isnt a decent tool to address it.
You just hit a particular peeve with a sideways glance.
There is a fundamental problem with "person B is on vacation and asked person C to cover". In the OpsGenie world, shift overrides are manual. Only manual. (I consider poking API with pre-calculated data as manual, too.)
What I really want is for OpsGenie to add support for concept of vacation. When someone is going on a vacation, it should be possible to tell the alerting system that during timeframe T, person P is unavailable - now recalculate and reschedule the rest of the roster. But no. They don't do that.
Our guys have asked for this feature during contract negotiations. I have asked it personally from their engineering staff. It's just not a feature they want to support.