While India is still a developing country, there’s obviously a gap in hygiene for India to improve. Basically, the public toilets and drinking water are not very clean in India. I’d suggest to bring hand sanitizer, tissues and wipes with you. Also, only drink bottled water and not a random cup of water. Many public toilets in India are like those in Taiwan 20,30 years ago, which only have a hole on the ground and usually don’t come with toilets paper. I normally try my best to avoid using toilets outside. I personally think this is one big problem in India.
About vaccination related issues, I’ve never had vaccinated in advance of traveling to India. Same as my mom and my sister, and all of us never had any health problem in India. I personally think as long as you're careful with the hygiene when you’re in India, you shouldn’t have any problem. For those who have weak immune system or sensitive stomach, you can refer to your national health bureau for detailed information about travel vaccination.
Another serious problem of India is air pollution. I have once got serious allergic reaction in India when my double eyelids turn to single eyelids. The symptom went away the next day after I took allergy pills. Hence I’d suggest you to bring your allergy pills if you’re also sensitive to air quality.