to add to tone-uncage-push the button:
On the commercial side, Vd is dive velocity. I've been to Vd several times, in several models, and even a knot or two beyond (depending on the recovery technique). I ain't destructed yet (much touching of wood, crossing of fingers, hail marys, etc.).
According to FAR 29.1505, Vne is defined as
"i) 0.9 times the maximum forward speeds established under § 29.309;
(ii) 0.9 times the maximum speed shown under §§ 29.251 and 29.629; or
(iii) 0.9 times the maximum speed substantiated for advancing blade tip mach number effects under critical altitude conditions. "
29.309 sets a speed based on structural considerations, which would include loads in the control system. 29.251 and .629 set a speed based on freedom from flutter and vibration anywhere in the helicopter.
Every civil certification agency I know of will only accept so much analysis for these speeds. In the end, yah gotta go fly it. This is Vd.
By the way, Transport Canada gets really picky, and won't accept that Vd is 10% over the Vne you finally get. You have to fly to 1/.9, or 11% over Vne.
Any manufacturer can actually select a lower target Vne, as Nick discussed before. This gives you a lower Vd, and you can still say that you have established a maximum speed under 29.309 et al, without getting anywhere close to destructing.