With whom does the child live, both parents and one is your child? Or only one parent, and if so, is the one parent your child or your in-law? It matters because if it is your child you will have a better chance of frank conversation than if it is your in-law. Ask what is going on, but in a concerned way. Sometimes people get overwhelmed. Sometimes grandmas are mistaken also, though a bottle that stinks is hard to miss. (Why bottles at 16 months?) Ask if you can have the baby for a few days and see how that goes over -- an exhausted parent might leap at the chance, while a parent who still has a lot of energy for the job might not. You could report your concerns to someone official if you are certain they are serious. Take pictures and document everything before you do.
Maybe you should help and support the mother. Maybe the mother is struggling and is doing the best she knows how. Try talking to her without being accusatory. Don't make it sound like your criticising. Find out why he hasn't been vaccinated, and offer to take him yourself. This may not be intentional neglect, it's probably that the mother just doesn't know, or understand, any better.