To add to Nick's excellant reply:
Most manufacturers will select maximum demonstration speeds (that then determine Vne's) fairly conservatively. Nobody wants to destructive test the whole friggin' helicopter - and crew.
To return to the original question, some of the theoretical limits used to set these conservative max demonstration speeds include advancing blade mach number and retreating blade stall. So, in a sense, the value of Vne may be based on these factors. A more correct way of stating it may be to say that the designers aim to get all relevant speeds (Vh, Vne, demonstration speeds, etc.) to fall below the point where things like tip speed or blade stall become of concern.
In the commercial world, Vh is often limited by operating cost concerns (how much engine and how much gas the customer can buy), and Nick is correct to state that Vne is then set some small margin above Vh. This then gives the designer some of the boundries to work within when selecting the parameters that will determine the rotor's high speed performance (airfoil, blade twist, etc.). Other boundries will be set by things like target costs and weights, and selected blade manufacturing method.
So, it's kind of a chicken and egg thing. Yes, blade stall and mach number problems may occur above Vne, but thats because Vne was picked to be below these (and other) problems. A good general definition of Vne would be the highest speed at which no speed-related problems are expected. Hope that clears things up!
By the way, for several after-market bubble window installations on AStars, Vne is set at 10 % below the speed at which the window blew off on test.
