Just to get all the information on the same page.
Backups using VIX Library instead of network access have one drawback: No Application Aware Processing without modifications to the virtual machine.
Application Aware Processing can be enabled by using an account with SID 500 (local or domain administrator, a big no-no in my book) for the backup or by disabling UAC in the virtual machine.
This does not mean that the backups are inconsistent! The backups will use the VSS writers and properly quiescence the filesystem and/or applications in the virtual machine. The only thing you're missing out on is the log truncating (the script that normally takes care of this is unable to start because of UAC)
tested and found to be valid