Hi alessandro,

One of the purpose of deploying VSS is so you don't have to use spanning tree any more, because the 2 physical boxes become logically one and you don't even have access to the second box in VSS mode. Spanning tree is not going to work, because the VSS pair is logically one device and so there is no loop.  You should also use portchannel on your downlinks ports to your access devices, if not in a case of a primary VSS failure the traffic may not fail over to the other uplink. The portchannel does load balancing for you and you don't have to divide your vlans into odd and even anymore.

HTH

Reza