Some vector tile services are built using references to multiple other vector tile services. Please see example which has 5 sources:
As of v200.4.0 ArcGIS Maps SDK support consuming such online vector tile services (using ArcGISVectorTiledLayer). No problem there.
With regards to exporting such a vector tile service to VTPK (using ExportVectorTilesJob) only the first source is exported (tried and tested).
With regards to consuming a VTPK indications are that it fails loading such a package (tried and tested by manually crafting a package with multiple sources). Please see Consume a vtpk with multiple sources in Field Maps - Esri Community
NOTE: ArcGIS Pro supports reading VTPK with multiple sources.
I realize after posting that the generation of a VTPK is happening server side. Thus I should make a feature request for the ArcGIS Server team.
Still would love ArcGIS Maps SDK to be able to consume a VTPK with multiple sources.