I've been working in the development of an ontology to help users with GTFS files (and, if not, to create them) to make the data compliance with the EU Regulation. This project is SNAP, what I translated as Semantic National Access Points :-) .
The roadmap to get it was developing an ontology which you can use for free  from scratch or, having a GTFS file, convert it to Transmodel  through the use of the ontology and some mappings.
As far as I know this development did not reach to the identification of the vehicle in order to assign it to a trip_id (which in Transmodel is something pretty much complicated than a simple "trip") but Transmodel and NeTEx  had for sure a model to do it.
If you need extra information with this approach please don't hesitate to ask.