Hi Fred / Freddy 
In my deployment workflow I execute a combination of batch files and PowerShell scripts and shell scripts for Linux VMs. Most of these scripts are copied to the VM during the deployment workflow and executed almost immediately. In one of the batch files I run an the AV install .exe that is a silent install package. Here's what I do which may help you
1) In the batch file (lets call it avinstall.bat), for the AV install the line is:
ECHO executing C:\Software\SEP11_setup_servers.exe...
C:\Software\AV_setup_servers.exe
2) I copy (GuestOperationsManager or VIX) avinstall.bat from the vCO server to the Guest (lets say c:\postdeployments\avinstall.bat)
3) I execute this using "Run Program In Guest" with the following (using the GuestOps workflows): - Note there is an intentional space at the beginning of programArguments
programPath c:\windows\system32\cmd.exe
programArguments <space>/c c:\postdeployments\avinstall.bat
With this, vCO is waiting until the batch file (and thus the AV silent install) is finished.