Note that from 5.1, proper rolling upgrades are possible - i.e in the past (like upgrading 5.0 to 5.1) you had to upgrade VCS on all nodes at the same time (although your application could stay up during this time) and then you upgraded vxvm on each node.  But now (so in your example from 5.1 to 6.0), you can upgrade VCS one node at a time, which means you can upgrade the entire stack (VCS+vxvm+vxfs) on the inactive node, then switch service groups to this node and then upgrade other node(s) (possiblly sometime later, once you have verified your application works ok on the new upgraded stack).

Mike