Morning Derek,
Is it still the case that: "Public Explanation: At this time the only supported vector basemaps in the WAB basemap widget are the Esri default basemaps. If using a custom basemap it must be a raster, not vector, basemap." at WAB Dev version 2.17?
I have our AGOL configured to allow vector basemaps to be used in WAB Dev. However, this only allows for you to add them to an app as "Esri Default Basemaps". I would like to create a custom basemap configuration where I layer our in-house contour product on top of the vector World Topographic Basemap since the vector version of the World Topographic Basemap does not actually have topo lines.