These are the first interesting links that I found roaming the Internet:
The Wikipedia page for the Vectrex. Has some history and tells about the working of the device and the 3D imager and light pen.
Richard created the VecMulti and VecFlash cartridges. The first one seems to be able to host different kinds of cartridges, as it mentions 64K bank-switched games. Uses a Micro-SD card as the backing store.
The VecFlash is discontinued and seems a precursor to the VecMulti.
Has information on the VecDev developer cartridge and some modifications like the resetting after uploading a game to the cartridge. Cartridge is supposed to be available from Vectrex.biz but it is not mentioned there I think.
Also, under Documents/Data sheets it has a lot of technical information on the electronics inside the Vectrex. Most importantly I found the Vectrex FAQ here, plus the programming manuals and a tutorial.
- Vectrex museum http://www.vectrexmuseum.com/
Complete overview of the original, prototype and homebrew games. Interesting news section. Also has a section with programming documentation.