timing rotation of a vector. math help?

[PrimerElite]

i'm trying to work this out in my head and i'm not quite sure how to get it down on paper. let's say i have a vector that defines an objects current direction. Now let's say i have another vector that defines the direction to another object. I wish to rotate the direction vector 20 degrees closer to the vector of the other object every second until it takes less than 20 degrees to rotate to equal the direction of the other objects vector. Can someone a little more mathematically inclined assist me?