If you want to initiate a TightVNC session from PDQ Inventory you can always configure VNC in the Preferences window. However, if you want to customize the session (such as automatically sending the connection password) you would need to use Custom Tools.
Go to Preferences > Custom Tools. Click the Add Tool button. Enter the the path to the VNC viewer file. After the path add your command line parameters. The example below would open a TightVNC session on the selected computer and it would pass in helpdesk as the password.
"C:\Program Files\TightVNC\tvnviewer.exe" -host=%TARGET% -password=helpdesk
When you select a computer go to the Tool menu and select the new custom tool and the session should be opened (assuming the target has the TightVNC server running and the password is correct).