I would consider not making VV10 throw an error, since it has negligible effects on the electron density. Najibi and Goerigk write in doi:10.1021/acs.jctc.8b00842

While in a strictest sense, energy calculations with vdW-DFAs should be carried out fully self-consistently, we demonstrate conclusively for a total of 11 methods that such a strategy only increases the computational time effort without having any significant effect on energetic properties, electron densities, or orbital-energy differences.

The recent analytical frequencies study by Feng, Liang and Head-Gordon in arXiv:2208.14639 states

The contribution of VV10 to simulating harmonic frequencies is shown to be small for small molecules but important for systems where weak interactions are important, such as water clusters.

That manuscript does have analytical second derivatives, so I guess one could extend it to TDDFT as well?

Psi has a mechanism to make VV10 a post-iteration correction. How about allowing stability analysis / TDDFT if VV10 is post-iterative, and raising an error otherwise, directing users to make VV10 post-iterative?

I think it is a bit of an open questions how to exactly incorporate a VV10 dispersion treatment for excited states.

Converted this PR to a draft. I do no support merging this PR while a LibXC bug is under investigation. (Hat tip to the LibXC team for that investigation.)