Many learned Hs (who focus on Vedic lit) find modern H emphasis on vegetarianism overdone, as the earliest literature of India is not big on it

But modern Hs are big on it

Largely because of post-Vedic theistic movements - Vaishnavism, some forms of Shaivism + Shramana impact

One is not sure when Vegetarianism became an ideal among the respectable elites of India.

Clearly by 300-400 CE, the ideal was extremely well established

When Fa Hien visited Pataliputra circa 400 AD, he was categorical in stating that most "respectable" people abstained from meat

So it definitely was an ideal by then

In Harivamsha section of Mahabharata (perhaps a work dating to the centuries before the Common era, but likely recalling earlier legends), there is a sea-side carnival of the Yadavas

In which the brahmanas abstain from meat. Not the rest of the party

So it seems the ideal was established among V1s at least by 300 BCE or so

As we do see Manu Smriti also praising abstinence from meat - a text that is usually dated to the Shunga period (1st cen BCE)

In the Mahabharata, in the Narayaniya section (Shanti Parva), performance of yajnas without animal sacrifice is praised if I am not wrong

Now dating the Narayaniya section is tricky. But most likely a work of the Mauryan-Shunga period, if not earlier

There's also Bhagavad Gita, where Krishna says I will accept a leaf if it is offered with devotion. Encouraging vegetarian offerings

Again a work some 3-4 centuries prior to Common era, if not older..

But if we look at Samhita literature, there is little to back "vegetarianism" as an ideal..

So it appears to have become one between 600 BCE and 200 BCE