I'm wondering what is the practical limit for the number of ESX host servers managed by a single instance of Virtual Console?
I am pretty sure the limit is around 250 ESX servers, whether or not you want to manage that many is another matter.
If you are using VC in a VM, that could be part of the performance issues. There is a distinct difference and don't let anyone tell you any different.
There is NO substitute for running VC properly on a physical host. That's how it should be run, and the mass majority on here don't run VC any other way.