The dot product of two parallel vectors is equal to the algebraic multiplication of the magnitudes of both vectors. If the two vectors are in the same direction, then the dot product is positive. If they are in the opposite direction, then the dot product is negative.
If A and B represent two vectors, then the dot product is obtained by A.B. cos q, where "q" represents the angle between the two vectors. Thus, if the vectors are anti-parallel, q equals 180 degrees; cos 180 equals -1. If the two vectors are parallel, then q equals 0 degrees; cos 0 equals 1.