I got heaps of documentation from an ex-DEC field service engineer. Among them there were a VT420 print set. I didn't see any schematics for the VT420 on bitsavers so even though this one looks a bit strange it is better than nothing.
http://storage.datormuseum.se/u/96935524/Datormusuem/DEC/VT420-engineering-drawings.pdf /Mattis
