I hardly dare write in the company of such distinguished experts, but there is a point here which is ATPL exam referenced, rather than pure engineering.
All agree that Va is a design speed used for calculation of stress effects in certification. In its form of Vs times the square root of the maximum positive load factor (normally 2.5g for JAR25 aircraft)it is a fixed value for design purposes, and Vs is the clean stall speed at maximum certificated takeoff weight.
However, if Vs changes, the calculated speed for Vs x sq rt N will also change. This does not mean that the design Va changes, it means that in practice you will meet the damage/stall point at a different speed. The ATPL examiners, however, have a series of questions asking how Va changes for different weights, using the term Va for the practical damage/stall speed. The answer is, of course, that this speed will change by exactly the same percentage or proportion that Vs changes, the square root of the weight change, or, for small weight changes - up to 20% - half the percentage weight change. Dick W