Since its major axis reaches about 0.6" at opposition, a 12" should have the resolving power necessary even in Red light.
To test this, I took pictures of both Vesta and a star of comparable brightness (SAO99128) that was very close to the asteroid.
I first processed the Red image, and it was very suggestive of resolution, the Green and Blue images are even more strong evidence, notice how the star's diffraction disk shrinks with decreasing wavelength, while Vesta stays the same size. Also notice how there appears to be elongation in Vesta's image, consistent across wavelengths but most obvious in blue light.
So I feel pretty safe to say that I not only resolved Vesta's disk, but am actually detecting the fact that Vesta is not a sphere, but an ellipsoid.