Solution
When opening the NI Vision Assistant from Windows menu, the software will always pop-up the NI Vision Assistant - Welcome window that request you to select a target to work with. In order to have the Create LabVIEW VI option enabled you must select LabVIEW as target.

If you want to change the code generation target from an existing vision assistant script (.vascr), you have to go to File>> Preferences option and change the Target for Generated Code option to LabVIEW to enable the Create LabVIEW VI option.
- When you open the NI Vision Assistant from a vision assistant script (.vascr) the NI Vision Assistant - Welcome window will not be shown.

Once you have selected LabVIEW as code generation target, you can verify this by checking the window title.