Maneuvering Speed
Va is not static, it varies with the gross weight of the airplane. Va is a limit that corresponds to allowable G. The formula to compute Va is: Vs x the square root of the design limit G. For example, if Vs=55 MPH IAS and the symmetric G limit is 6, then Va would be 135 MPH. Keep in mind that IAS associated with stall changes with gross weight (critical angle of attack remains constant). So at lighter weights, Vs DECREASES as does Va.
Another thing to keep in mind is difference between symmetric and asymmetric G. "Symmetric" maneuvering is pulling about a single axis. A loop or split S are examples of maneuvers about a single axis. Assymetric G occurs with simultaneous maneuvering about two or more axis. Any maneuver that requires roll AND pull is "asymmetric." A barrel roll is a familiar example. Asymmetric G is sometimes referred to as "rolling G" but this is a bit confusing since some rolls are symmetric. If, however you are "pulling" and then input aileron (or vice versa), you are maneuvering about two axis. Vans doesn't specify asymmetric G limits but we can look to MILSPEC or FAR 23 for guidance.
MiLSPEC reduces allowable G under asymmetric conditions by 20% and FAR 23 stipulates a 4G asymmetric limit in the aerobatic category. Manufacturers can, of course specify higher limits, but since no limits are specified, the most conservative ESTIMATE would be the 4G FAR 23 specification. So in the first example, Va for an asymmetric maneuver (e.g., snap roll) would be 110 MPH IAS (55 x the square root of 4).
Summary: Va is a fluid number that is affected by gross weight and G allowable. G limits depend on gross weight and whether or not manuevering is symmetric or asymmetric. If you know IAS for stall at varying weights, you can compute IAS for Va based on G limits. G limits are specified by Van's (although asymmetric limits must be estimated).
Fly safe,
Vac