The answer is that Esri will support this better, hopefully soon:
https://community.esri.com/t5/arcgis-online-ideas/vector-tile-popups/idi-p/972445/page/4#comments
Where you can have a vector tile layer in your map and query a feature layer outside the map. It's currently possible in Pro, but not the Web Map Viewer.
In your case I think adding both a vector tile and a feature layer (no symbology?) is not best practice. You'll have all the weight of a feature layer in your map anyway. Just use the feature layer.
18mb is not a large feature service either. I wouldn't expect drawing speed issues nor a need for a vector tile layer. It could be that this layer could be optimized to draw faster. Try:
- Optimizing the layer to improve feature performance
- Have the feature layer in the same projection as the basemap, to avoid projection on the fly
- Disable editing
- Don't use relative queries (e.g. date-based)
- Check the geometry in Pro, particularly for lines and polygons. Too many vertices? Run the generalise tool. Run check geometry to see if there's any issues.
In summary it will get better soon, but for now if you need feature layer functionality, use a feature layer and take the steps to ensure it's performing optimally. If you don't need feature layer functionality like query or editing, just use vector tiles.
