Bingowings said:

Strictly speaking none of Anakin's hilts are the same as the one Ben gives Luke (which changes from between and within the OT anyway).

Vader's OT hilts (and they change too) clearly evolve from Anakin's so you could argue that he just adjusts his first Sith saber over time until it looks like the way it does in ANH.

The general idea is that in terms of narrative Darth Vader has a saber that Anakin Skywalker doesn't use and his son carries the sword of his father (not his father's 'murderer').

The blade colour is the thing that is going to catch the eye of the audience (only the mega nerds notice all the continuity screw ups with the hilts anyway).

If you couple that with Anakin's blade being retrieved from somewhere other than the planet of fire (i'm sure there are other potential ideas other than the ones I've postulated) and you have an alternative explanation for where Luke's sword comes from and which sword is which.

Wait....

We AREN'T mega nerds? :O