The VecAdapt is a hardware interface allowing the use of an unmodified Sega Mega Drive (or Genesis) controller with the Vectrex. Fifty units were produced by "computer nerd" Kevin Koster and were released for sale to the general public on Dec. 4, 2014. 
Advantages to using the Vectrex-to-Mega Drive/Genesis Controller Adapter Edit
Mega Drive = Sega Genesis in the USA EditA note on the name: the Mega Drive was released in the American market as the Sega Genesis; both names refer to a compatible product simply rebranded. Because the name "Mega Drive" was already registered in America to a hard disk storage solutions company, an alternative was chosen. This article will refer to the console as it was known in the global market (the Sega Mega Drive) and and what it presumably would have been called in America if it weren't for legal issues.
Mega Drive controllers are still plentiful and inexpensive. With a 14-year retail lifespan, the console spawned a wealth of original and 3rd-party controllers. Plenty of inexpensive and also newly-produced generic controllers can be readily found on ebay as low as US$5 for a new (generic brand) controller shipped.
Compared to original working Vectrex controllers, this is a cost effective alternative to hunting for original units to add a 2nd controller, or just to save on wear and tear on valuable originals.
To adapt? Or modify? Edit
While Vectrex users have been modifying Mega Drive controllers for use with the Vectrex for many years, the VecAdapt is the first pre-built product allowing use of an unmodified Mega Drive controller which can be used interchangeably with both the Vectrex and the Sega Mega Drive / Genesis. The advantage here is that once the a controller is hacked for Vectrex use, it will never work with the Sega console again. The VecAdapt solves that problem and is ideal for households containing both Vectrex and Mega Drive systems. Also, it's easier for the less DIY-inclined.
Basic setup Edit
The basic setup requires three steps:
- Plug the VecAdapt into the Vectrex controller port
- Plug an unmodified Sega Mega Drive controller into the VecAdapt
- Start playing!
Tony Holcomb also pointed out in the pre-production discussion thread that having dedicated controls is preferable to controller modding "not only because you can use them unaltered and still use them on the system they were originally intended for but also because you can use whatever controllers or accessories were made for that system." It might open up the possibility that accessories like the Justifier, TeeV Golf or Batter Up Bat could be designed to work with the Vectrex. 
Additional Features Edit
The Vecadapt has two slide switches on the front, one to enable a built-in rapid fire circuit, and one to change the order of the buttons.
The VecAdapt is available directly for $38 AUD plus postage from Kevin Koster's webpage computernerdkev.heliohost.org. Including postage, this comes to around $44 USD for a single unit shipped to the United States.
Alternately, an additional $10 AUD can be paid for the privilege of ordering through ebay. See user the-mercuryarcrectifier.
Other editions and alternativesEdit
Pre-production special editionEditThe first three "pre-production" units were housed in a wooden box with the word "VECADAPT" stenciled in large letters on the top.
The next 50 units were built in plastic project boxes welded shut with plastic adhesive. They are labeled with printed paper with the serial number individually printed on each one.
At the time of this writing, in 2015, VecAdapts were still available from the initial run. Even once the units have been discontinued, schematics are available for resourceful DIY'ers. See: VecAdapt Technical Information for instructions, although Kevin notes in his original post that he'd prefer if the user bought one of his.
Earlier Mega Drive controller-to-Vectrex conversion projects Edit
Using Sega Genesis Controllers with Vectrex (Revision 3.1) , 1995, Brian Holscher.
Sega Six Button Controller Hardware Info (Revision 1.0), 1996, Chuck Rosenberg.