From the Century Dictionary definition for vector:
"In quaternions, a quantity which, being added to any point of space, gives as the sum that point which is at a certain distance in a certain direction from the first. Vectors are said to be equal when their directions and magnitudes are the same. Unit vectors in quaternions are considered as equivalent to quadrantal versors having their axes in the directions of vectors; the word vector has accordingly sometimes, but incorrectly, been used in the sense of a quadrantal versor. Every quaternion can be resolved in one way, and one way only, into a sum of a scalar and a vector; and this vector is called the vector of the quaternion, and is denoted by writing V before the sign of the quaternion. Thus, Vq denotes the vector of the quaternion q."