@alexredston Hey, sorry I'm a little bit late here. So, with regard of VCPUs - there's a hardcoded limit of 128 actually in XEN hypervisor. Moreover XEN toolstack (when creating a guest) will check that guest VCPU limit is below physical CPUs available on the platform.
Bypassing the VCPU 128 limit will require some rather important adjustements in XEN Hypervisor (basically the restrictions go with IOREQ server from QEMU and how LAPIC id are affected in XEN). So with the next XEN version this limit could potentially be increased (there's an ongoing work on this).
The things you also probably would like to know about this VCPUs limit
not all of the guests can handle this VCPUs number (e.g. Windows will certainely crash) when you gives a VM such a big VCPU number (basically more than 32) the VM can potentially provoke the DoS on the whole platform (this is related how some routines are "serialized" in XEN Hypervisor). So, when you do this - be aware that if your guest is broken, pawned, whatever... your whole platform can potentially become unresponsive.