Not sure if this was an anomaly but I was using both /clean and /uninstallagents on a single aexagentutil command-line in my batch file to completely remove all remnants of Altiris 6 where I tried again by only using the clean switch. Note that I chose this complete removal option to avoid the known side effects of redirecting my Altiris 6 agents to my SMP 7.1 server.
After pushing the SMP 7.1 SP2 agent to my fresh new test VMs (i.e. I recreated/cloned them all from Sysprepped base templates, pushed the Altiris 6 agent onto them then ran my batch file to completely remove the client) and inventory was collected, they all properly show up under the Virtual Machines view now!
I found KB docs that seem to indicate only 1 switch should be used with aexagentutil so perhaps my initial use of 2 switches in my script somehow affected the resource type tagged onto 2 of my 3 VMs that identified them as computers instead of virtual machines.