I would guess not.
If the answer is no. How do you know that?
Because even a tiny index of refraction would induce a gigantic chromatic aberration in the light of distant sources such as quasars. An index of refraction doesn't just slow the speed of light; it slows it in a frequency-dependent way. If that dispersion accumulates over cosmological distances, I don't see how we'd miss it.
The three dimensions of lens design =
Indexes of refraction of the lens materials
Dispersions of the glasses
Curvatures of the lens surfaces and air spaces.
These variables are used together to attempt to reduce blurriness by bringing all rays of all colors from an object point to the same focal point. Then you also have axial rays and off-axis and non-parallel rays to deal with. When you have a good solution you can then proceed to ruin everything by making it a zoom lens.
This is true for any medium where the dielectric constant is greater than 1.0. But this is a different case since we are talking about the dielectric constant of the vacuum. That is the problem. Could a pure vacuum have a propogation velocity that is frequency dependent? If so, you are right, things would look very odd. Everything would be rainbow. But I don't think a pure vacuum could behave that way even if the propogation factors were different than we now believe.
But I am just guessing.