I'd say this is definitely a question for the vet. A million things can cause a cat to vomit, and waiting/guessing/trying home remedies can allow a condition to worsen to the point that the vet bill would be MUCH higher, and maybe come too late to prevent permanent damange. The vet has a much better chance of figuring out what's going on (germs? wrong food? ate something inedible? allergies?) and getting Simon feeling well again quickly.