Using the "Add layers from web" workflow, you should be able to enter the URL for the corresponding Vector Tile Layer into your web map in Portal for ArcGIS, and load the layer.
To find which URL to use for the vector tile layer:
I hope this helps!
Thank you for the reply Thomas, and yes, that does seem to be the right way to do it, but the urls provided for the sample vector tile layers are pretty generic. For example, the modern antique sample url is listed as:
(I think that's the base data and doesn't include the styling laid on top of it?)
If you attempt to add that url to a Portal web map as you described above you get the following error message:
The layer is not a supported type and can not be added to the map.
After to talking to some folks at ESRI, I was told that this functionality is not yet supported for Portal for ArcGIS yet.