Now you can make a video call directly from google assistant. Before that assistant wasn’t able to send a message on Allo or start a video call on Duo.
To use this feature you have to say “video call contact_name” and it will make a video call automatically. But this feature is not available to all users right now. If you don’t have Duo installed, then Assistant will use Hangouts instead. You can update your Google Assistant to the latest version from google play store.