In the engine-out case, what is the difference between Vx and V2? Don't both achieve the best possible gradient for the configuration?
In that particular configuration they will probably be very close if not the same. However by definition they are two different speeds and need not be identical. V2 speed is designed to provide maximum obstacle clearance at take-off with one engine out, it will probably not be the speed that will give you maximum angle of climb in the long run, which is Vx (and probably a higher speed than V2 I would think).
I do find it strange how many larger aircraft do not have published Vx and Vy speed, as they are so useful to have knowledge of. With all the other flight tests those aircraft go through I can't imagine it would be difficult to determine Vx or Vy, so why not just add it in the books!?