We too had a difficult time with modified vector tile display in our ArcGIS Online org when it was at the 10.5.x equivalent, so much so that I left it alone for some time.
We have since moved fully to the Enterprise and are at 10.7.1 and the workflows mentioned here do work and we now have a modified style of the Light Gray Reference layer (although not the new WGS84 style) that I can use as reference layer on top of my own vector tile base layers. It works well in the portal map viewer and in Web Application Builder's (2.13) basemap gallery widget.
HOWEVER, the modified Light Gray Reference layer (that I am calling our Local Gray Canvas Reference layer) will not load as a layer into ArcGIS Pro at 2.4.2 without error. Not sure why. In my portal, the modified vector tile shows as coming from my https://myorg.net/portal/sharing/rest/content/... directory, but the source url of the modified tile layer still shows as coming from the esri source at https://basemaps.arcgis.com/arcgis/rest/services/World_Basemap_v2/VectorTileServer
Don't have a workaround yet. It kinda feels like a CORS issue.