@BrettLessner- Agreed. If there's a GP tool or some API, I'm not aware of it, so it looks like the current map extent is the only way to set the extent.
If you have to download it in multiple extents to get the scale you want, you want tiles that are non-overlapping. Many of the polygonal features, like parks, use partial transparency to display a hillshade behind it. Any overlap will really stand out. I found some Python code to map the extent of a map and it ran, but did nothing. Not sure if I was doing it wrong, but I couldn't get it to work. Very similar code applied to a layout worked. If you activate the layout, the download map function is enabled. Here, I have downloaded the map for the current map extent and pan the map to the right/east by exactly the width of current extent to get non-overlapping extents.
# Activate the map frame in the layout you will use.
# Save vector tiles for current extent before running this.
aprx=arcpy.mp.ArcGISProject("CURRENT")
lyt = aprx.listLayouts('VTPK download and clipping')[0]
mf = lyt.listElements("mapframe_element", 'VTPK download and clipping')[0]
extent = mf.camera.getExtent()
width = extent.XMax - extent.XMin
extent2 = arcpy.Extent(XMin = (extent.XMin + width),
YMin = extent.YMin,
XMax = (extent.XMax + width),
YMax = extent.YMax,
spatial_reference = extent.spatialReference)
mf.camera.setExtent(extent2)
# Now save the vector tiles for the new extent
As far as I could zoom in on screen, everything seamed together perfectly.