Hehehe 🙂
It should be getting stitched, provided you are calling the visitor id service correctly with the visitor ID code...
I have a subdomain that I cannot add launch to, I am using AppMeasurement.js implementation with the Visitor ID service.. users move from the main site (using launch) to the subdomain (using AppMeasurement and VisitorAPI) then back to the main domain... The users are all maintaining the ECID throughout this journey, so I know this is possible....
The ECID is being set by the service, not the code... even if the versions are slightly off.... even to complicate matters... I have three domains going through the same single subdomain (they didn't want to provision out a separate instances) - so the majority of the flows stay on the same domain root - meaning shared cookies and the ECID that is already set can be read - but the other two lower traffic flows may need to set the ECID again when moving from domain A to B, but it's still setting the correct values.... and stitching the user together so that I can see their journey.
Note: I am using a global tracking suite for ALL the mentioned domains... so same org id and same suite....