\vector(x-slope,y-slope){length}
The \vector command draws an arrow of the specified length and slope with the arrow head at the opposite end of the line from the reference point (see \put).
The slope is determined by (x-slope,y-slope) which are signed integers of magnitude less than or equal to 4 and which have no common divisor except for plus or minus one; For example (1,0) is a horizontal arrow pointing right; (1,1) gives a slope of 45 degress; (0,1) is a vertical arrow pointing up; (-1,1) is at 135 degrees.
The horizontal extent of the arrow is given by the length parameter, except in the case of a vertical arrow in which case length specifies the vertical height.