Hi Paul,
You aren't doing anything wrong, Popups on Vector tile are not supported in the ArcGIS online map viewer yet. It will be coming soon, Here are some discussions regarding this support:
- Vector Tile Popups
- Can you have attributes display for Vector Tiles?
As the ArcGIS Pro documentation suggests under Retain Relevant Attribution
"To bring specific fields into the vector tiles, highlight them in the Fields view of the layer. Highlighting the ObjectID field adds feature-level identity to the vector tiles and allows for pop-ups to be viewed from the tiles themselves"
We have a recent enhancement that has asked for the Pro Documentation to note the online support until its functionality arrives.
That record is ENH-000119818.
Please feel free to contact support to have your account attached to the record.
Hope this helps
-Adam