I am certainly not some anti-vaccine nutter. however, are covid-19 vaccines less likely to be efficacious in protecting people with multiple sclerosis?

This is NOT correct, and people with MS would be excellent candidates for the vaccine. But some of the medications are either immunomodulators or immunosuppressants, and these could block the vaccine from providing the full benefits. Each case of MS therefore must be assessed based upon current medication, and some patients have switched treatments due to need for vaccine.

The degree to which an antibody response develops is in part related to the trength of the individual's immune system. An example of this is that Moderna vaccine can provide 95.6% immunity in people aged 18 to 65 – but 86.4% in those over 65. REF: https://www.bbc.com/future/article/20210114-covid-19-how-effective-is-a-single-vaccine-dose