I haven't used proxmox so I'd have to guess.
From their web page and not very well written. http://pve.proxmox.com/wiki/Network_Model
Proxmox VE uses a bridged networking model. Each host can have up to 4094 bridges. Bridges are like physical network switches implemented in software on the Proxmox VE host.
All VMs can share one bridge as if virtual network cables from each guest were all plugged into the same switch.
For connecting VMs to the outside world, bridges are attached to physical network cards assigned a TCP/IP configuration. For further flexibility, VLANs (IEEE 802.1q) and network bonding/aggregation are possible. In this way it is possible to build complex, flexible virtual networks. "
Looks like you kind of have a two step process. It is not a bad question at all. Maybe the proxmox forums would have a solid answer also.