Vectrex Controller

9 PIN UNKNOWN CONNECTOR

Pin	Description
1	Button 1
2	Button 2
3	Button 3
4	Button 4
5	Horizontal Pot
6	Vertical Pot
7	+5V
8	GND
9	-5V

The joystick potentiometers work by voltage division between -5V and +5V. Actually, it uses a couple of resistors on each side to make it more like -3.4V to 3.4V.

Vertical Pin 6:

Voltage	Direction
-3.4 V	Down
0 V	Center
+3.4 V	Up

Horizontal Pin 5:

Voltage	Direction
-3.4 V	Left
0 V	Center
+3.4 V	Right

Contributor: Joakim Ögren, Jay Tilton

Source:
Vectrex Controller Pinout at GamesX
Jay Tilton's Website