I have two cats and they have problems with vomiting?

they frequently vomit hair, food, everything! its horrible they always clean each other so the hair build up i've tried the petroleum stuff the vet gives you in the toothpaste like tube. i've tried shaving them (which helped a little) and brushing them. they currently eat purina cat chow adult they are 4 yrs. old and its always been a problem. i dont know what else to do any ideas?