Scalar is implied by the piping code. B31.3 (302.3.6(a)) states the sum of longitudinal stresses ... due to sustained loads ... and of the stresses produced by occasional loads ... may be as much as 1.33 [Sh]. Other Codes with equations show the stress magnitudes summed, not stress vectors.
Why? I figure many occasional loads have a "rebound" component so if you say the pipe moves 25 mm in +X, it might just as easily be 25 mm in -X a moment later. Just my assumption. To be conservative I figure those occasional loads are + and -.
Of course, CAESAR II will allow you to do different if you wish. That's between you and the owner.