I've just tried with my ESXi 5.1 test box and works fine with a local user, don't need su - or sudo.
~ # vim-cmd vmsvc/getallvms
Vmid Name File Guest OS Version Annotation
1 test1 [datastore1 (1)] test1/test1.vmx windows7Server64Guest vmx-09
2 test2 [datastore1 (1)] test2/test2.vmx windows7Server64Guest vmx-09
3 test3 [datastore1 (1)] test3/test3.vmx windows7Server64Guest vmx-09
4 test4 [datastore1 (1)] test4/test4.vmx windows7Server64Guest vmx-09
Have you assigned proper permission to the user? I've assigned Administrator to it for the testing purpose.