V_A, or maneuvering speed, is determined by calculating the speed at which it is possible to exceed the g-force rating of the aircraft. At speeds less than V_A, the wing will stall before excess loads can be applied to the airframe.
For example, if we imagine a 1000 lb aircraft with a positive load limit of 3 g, flight slower than V_A will mean that if the pilot tried to pull up at 3 g, the wing would be unable to produce 3000 lb of lift without exceeding the stall angle of attack. As speed increases, the angle of attack for a given amount of lift decreases. There is therefore a speed at which the wing can support 3000 lb just before it stalls. This is V_A. Above this speed, the pilot could develop more g force than the airframe is designed to handle (which is bad).
The slip is a one g maneuvre, so I can't see it being important whether you're above or below V_A. I would be somewhat cautious at high speeds, however, only because you can place large loads on the airframe with high speeds. As such, I'd move in and out of the slip smoothly. I'd also stay a fair margin below red line as your airspeed indicator will read incorrectly in a slip.
The Airbus tragedy referenced earlier isn't relavent to smaller aircraft, from what I understand of it. I believe the limit on full rudder deflection was a known "feature" of the design and was discussed in the flight manual or other documentation from the company.
Chris
