No it does not have to 2x 10Gb interfaces. You can use 1 physical interface, but that must be part of a port-channel as the " vpc peer-link" is only available on a port-channel. That makes sense, later on when you want to add additional interfaces.
The question is would you want to use 1x 1Gb or 1x10Gb interfaces? For that consider what traffic in your network travese the Peer-link and ascertain if the link size will suffice for the amount of traffic.
This traffic would include but is not limited to:
- VPC control and synchronizing traffic.
- Traffic to Orphan ports.
- in failure scenarios can the peer-link carry all relevant uplink traffic if it can only exist via the peer chassis?
- etc.
In a split brain scenario, when you loose both your peer-link and peer-keep-alive link, both systems will assume themselves as the only operational system, and will forward traffic. This should (and I say generally) not pose a problem for STP or data traffic. (i.e., all traffic received on a downlink port can only exit on an available uplink port). Both chassis will appear to other switches are one switch since they both will be sourcing traffic from the same virtual system VPC mac.
Alterntive have a read trhough the N5K design guide, and check out the VPC failure scenarios.
http://www.cisco.com/en/US/products/ps9670/products_implementation_design_guides_list.html
I hope I answered all you questions sufficiently
