Depends on how you set things up.
You can isolate CPUs from unRaid and dedicate them specifically to the VM, in which case unRaid will never touch them regardless of if the VM is doing anything or not.
Generally though, you would run Plex as an app on unRaid itself instead of within the VM, in which case whenever Plex isn't doing anything the OS can utilize the cores for whatever it wants to use.
And, many many possibilities in between.
IMHO, there is only one choice