fertility rate is falling in India for every section but at the same time one specific group still up in fertility rate and if we continue to follow this trajectory then big demography change is coming in next 20-30 years. Without sounding alarmist, one should remember what happens when some group reach 30-35% of total population or become majority.
I looked up the reports of fertility rate decline categorized by religion and all are declining compared to past, so I am not sure you are implying the religion or caste here. 
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But this is a bit stretch and even if it's real it can't be planned and it must be due to the natural diet practices followed by them and vegans tends to lose it more than the others while we still have government propagating religious views and it could influence some to change from what they are supposed to do.
This argument reminds me commentary of some pakistani cricketers, indians players don't eat meat that's why they are not strong lol. Diet etc is important but not the only factor. Education plays the biggest role. One example is this, you said it yourself "And for an average middle class their kid fee will be around 20-30% of their salary for a month and that is why now we are not interested in multiple kids just stuck with one."
Then there is another problem from illegal migration from Bangladeshi and Rohingya, it will certainly add upto the problem. Why do you think PM had to announce demography mission from the red fort ( focus was not fertility but the migration).