Get the input for both the horizontal and vertical axes of the stick you want.
angle = arcTan( stickY/stickX )
moveVector( cos(angle), sin(angle) )
Do whatever to make the code work. Using the arctangent from the stick data will get you the true angle, and using cosine and sine as the X/Y movement increments will ensure it moves in the correct direction. Increment at whatever units you're using.
Hopefully that helps.