Combine tile layers in VTSE into one style?

[SamGuilford]

I'm interested in importing my own boundary lines (as a vector tile service) into VTSE so I can tweak them along with a preset basemap layer and save out one overall style. Is it possible to do this, combine layers in VTSE? For instance, can you combine the preset base and label layers into one style? Couldn't find anything. Thanks.

[Andy-Green]

@SamGuilford I realize this is a long time coming with a response, but there have been improvements to the VTSE over time that have made this process a lot more straight forward.  As long as your hosted tile layer is in the same spatial reference as the basemap (we have Web Mercator Auxiliary Sphere and WGS84/GCS tiling schemes of both Esri Basemaps and Open Basemaps).  A good example of this is our World Topographic Map (with Contours and Hillshade) which has three separate services being called by one style.  The important thing is to make sure each tile service has its own unique name and your styling layers for each feature class uses the correct source name.  Then you can layer your style code for your boundaries at the correct positioning so they appear in the right drawing order of your new map style.
There's a blog with additional details:  https://storymaps.arcgis.com/stories/81c2d3a43fd449d59b99268f299d6cdc

JSON example of the Topo with contours and hillshade where the basemap (yellow highlights) is using the "esri" source name and the vector hillshade (green highlights) is using the "hillshade" source name in the styling layers.