Dogs were never naturally vegan. We cannot change the diets of animals based on our beliefs, for instance, a cow's grass food cannot be replaced with chicken. Similarly, we cannot rely on wheat (roti) for dog's food because it is not what dog species need for nutrition. There are many nutrients in different food items that different animals require. Vegan food items will be deficient in vitamin B12 and taurine. If you are giving your dog a vegan diet, you will have to add artificial supplements. It is extremely difficult for your dog's kidney to digest such vegan food items. It takes years of proper diet and nutrition to avoid problems in the future. Regularly feeding your dog a vegan diet will cause kidney problems later which might be irreversible. Having said that, if we feed our dog one meal of lentils, it will not be toxic or harm the dog in a poisonous way. The exception though are giant breeds. Giant breeds are more susceptible to issues like excessive gas formation which can cause the stomach to turn. It is important we understand that dogs need different nutrition than humans. Please do not stick to only a vegetarian diet. Dogs will need nutrition value from non-vegetarian food. Also, ensure proper rotation of food and meat to prevent many health issues. Dogs may also get over their protein allergies. Please check the digestible value and bioavailability your dog is getting from the food.