Looks like you're referring only to seat duration specs when looking at overlap, which will tell you when there is .006" of valve lift having a negative effect on vacuum. Think hard now, how much do you actually suppose this amount of valve lift effects things? It isn't very much
This is the main reason why the common spec of "0.050" exists. it is used as a point of reference where it can be considered that valve lift actually starts to perform it's function, allowing gases from the intake runner to enter the cylinder and for pressures between the two to equalize.
Put in simpler terms, how long the valve overlap period occurs can be seen with duration numbers alone. But how far they are open requires both seat duration AND duration @ .050"
We know that more degrees of overlap will reduce vacuum.
We also know that you lose more vacuum through a big hole than a small hole (like a completely open valve compared to a nearly shut valve. for an extreme example)
So if we have two camshafts with the same degrees of overlap as measured by seat-to-seat specs, but one having more overlap at 0.050". The one with more degrees of overlap @ 0.050" is reaching higher valve lifts earlier as they open and keeping them open for longer as they are on the closing side of the ramp, even if the total duration is the same. Since overlap happens at the beginning of one valve and the end of the other, this is an important factor.
Even with the same seat duration, a higher spec @ 0.050" will have the valves be open FURTHER during the overlap period and as such will have less vacuum at idle.
