Using strategic sampling noise to increase sampling resolution
Altering Vref would be a multiplicative scaling, that's not really going to be useful across the range of inputs in walking values across code thresholds a statistical fraction of the time as dithering attempts to do.
For dithering you want additive noise, not multiplicative distortion that is functionally gain variation.
Eg, you want to add various fractions of an LSB so that sometimes you hit the higher of the closest two codes and sometimes the lower, and then if you analyze the comparative rate of the two and assume your noise is random, you can over time calculate a more precise intermediate value to a fraction of an LSB.
In contrast, dithering by gain variation would only really work usefully for a small range of inputs, in the upper part of the allowable input range, lower in the input range the effect would be proportinoally reduced.