Vne
I recall that Vne is 90% of Vd - (not joking) - which is maximum demonstrated dive speed during test flying.
I don't know how Vd is calculated - I guess the design engineers figure out how fast the plane will hold together and the rest is up to the test pilot.
From engineering perspective, maximum turbulence penetration is a few knots less than Va due to centre of pressure differences - Va is to cope with hi-G whereas turbulence penetration is to cope with sudden vertical gust onset - can't recall the rest but is all in a book somewhere on my shelf.
However Va is close enough for all light aircraft.