In this case because people is plural, you do not need to include an article before "visual impairment" and "hearing impairment". What you do need to do is change "visual impairment" and "hearing impairment" to their plural form so the phrase becomes "mostly with visual impairments" and "people with hearing impairments".

I would say that is spoken English either is fine, though it is probably more common not to use an article for the reasons given.