You might try the workaround I used.
- Create an empty web map in ArcGIS Online which uses the Vector Basemap.
- Save that empty web map as something like "Navigation - Vector" in ArcGIS Online
- Sign into ArcGIS Online in Pro
- Open your "Navigation - Vector" web map.
Another approach - though I haven't tried it could be to save the Vector Basemap layer in your Webmap as a Layer file in ArcGIS Pro once you've done the above. Then just add that layer file to your web map anytime you needed it. That would theoretically allow you to delete the "Navigation - Vector" web map you created. Again, I've not tested this on my end but in theory, it should work.