Here's some useful Vectrex stuff:
Programming Manuals:
http://www.playvectrex.com/designit/fredtaft/vecprogman_v1.pdf
http://www.playvectrex.com/designit/fredtaft/vecprogman_v2.pdf
Vectrex Programming Tutorial:
Service Manual (schematics are actually useful, given how closely the software must interact with the video circuitry):