It shouldn’t be a problem.
As any parent who has breast fed knows, weaning is a gradual process. In all mammals a normal transitional period occurs. During this period the infant nurses and consumes other forms of food and water. Over time, a complete transition away from nursing occurs.
I am troubled by your reference to vomiting. Are the kittens vomiting regularly? If so, you should have a vet check them out. (Actually, by eight weeks of age, a vet already should have checked them out! They need vaccines and deworming medications.)
However, if your kittens are not vomiting, then it sounds like they are going through a normal weaning process.