FWIW, I'm fairly sure the priority.grabbed option wouldn't help anyway... it will only change the host-side scheduling priority of the grabbed foreground VM -- i.e. it can be used to give a priority boost to the VM you're using at a particular moment, above the VMs you're not interacting with. AFAIK, it has no effect on the decision of whether or not to grab into a VM, which is what's misbehaving here.
Cheers,
--
Darius