These are the first interesting links that I found roaming the Internet:

1. http://en.wikipedia.org/wiki/Vectrex
   The Wikipedia page for the Vectrex. Has some history and tells about the working of the device and the 3D imager and light pen.
2. http://www.vectrex.biz/
   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.
3. http://www.vectrex.co.uk/
   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.
4. 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.