Again agree with Lock n' Load.
What has to be remembered is that radar or lateral separations are designed so that provided the minima are complied with, there is practically no chance of a mid-air.
Separations for wake turb. are not deigned to ensure that the following aircraft will have no consequent effect. They are designed to ensure that when the other aircraft receives the effects of the wake turb., it's effects are reduced to an acceptable level.
To really look at this question, put your procedural hat on. You establish the pass point and confirm it by position report. You then count to 10 and swap levels. Wake anyone?
To make this question more interesting, make the light aircraft a C150 flying at 80Kt into a 50Kt headwind. Would that change your opinion? Waiting until the C150 is 6nm beyond [A] is going to take 12 minutes by which time the B767 will be some 100 miles past [A].
The wake of the B767 is created in the parcel of air that the B767 is passing through. That parcel of air in the case I have given is moving at 50Kt (the windspeed).
Let's put a helicopter at A, 1000ft below the path of the B767.
At the instant the B767 passes [A], the helicopter climbs vertically at 500ft per minute to the same level as the B767. It takes 2 minutes to complete the climb. With that wind, the instant turb encountered at [A] (ignoring sink etc) would be that which was produced by the B767 1 2/3nm before [A] (distance traveled at 50Kt in 2 minutes. That turb will have been subjected to disipation, divergence etc. (Remember that a light can take-off behind a heavy with 2 minute separation! and low speed =high induced drag = strong wake vortex!
So think of the helicopter at [A] if it travels in the same direction just behind at the same speed as the B767, it gets the full whack of wake. If it as 0 groundspeed (climbs vertically it gets a lot less).
If it travels at a significant groundspeed in the opposite direction.............
Regards,
DFC