Okay, rules as written time. Once a target ship has been hit by tractors (assuming it was hit by enough tractors to actually hold it, based on the size of the bay(s) and the displacement of the target ship), the target "cannot expend Thrust but may be moved by the tractor beam operator as if they had Thrust 1." If you're using vector movement, I doubt the intention was to "magically" match the target's vector to the tractoring ship, so I would rule that (as long as the target was within tractor beam range), the gunner was in control of how the target accelerates (up to a limit of 1G). Which does imply that with a high vector difference between the ships involved, the two are likely to move out of range (Short) of each other before vectors can be matched enough to keep them within range, thus breaking the tractor lock. (This illustrates the major problem in high-relative-velocity combat - you only get a small number of shots before you sweep back out of range, so if you want to keep after the other ship, you either have to match vectors or carry out a series of attack runs, between which the opposition may well have the chance to run, whether in real space or by jumping. This makes piracy even more difficult, among other things.)
Also as a side note, tractor beams cannot lock onto a target which displaces more than their own ship. (Which I don't personally agree with; this would make ideal equipment for the spacecraft equivalent of tugboats.)
