If your dog is a diabetic on insulin and is not eating
at least 1/3 of what they usually eat, then you should not give the insulin at all. If he is vomiting today, then hold him off of food and water until he has stopped vomiting for at least 6 hours. Do not give any insulin. Once you have reached the six hour point then you can offer him a few tablespoons of water. If he holds this down, then offer him a few more about 2 hours later. As long as he keeps the water down, you can then offer him a few tablespoons of a bland diet. I recommend boiled white rice and boiled white meat skinless chicken
breast, mixed 3 parts rice to 1 part chicken. If he holds this down, you can offer him another few tablespoons of it a few hours later. Slowly increase the amount of this bland diet that you are feeding
him over the course of 24-48 hours. Once you feel that he is back to his normal self, you can gradually mix in his regular diet with the bland diet until he is fully back on his usual food.
If he continues to vomit today (if the number of vomiting episodes totals more than 4) then you should take him in to see the vet immediately, as this is alot of vomiting and may be indicating a major problem, possibly one associated with his diabetic condition.
I hope that this information is of help to you and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.