As much as a founding principle as Hinduism can have. In his commentary on the Yoga Sutras, sage Vyasa defines ahimsa as "the absence of injuriousness (anabhidroha) toward all living beings (sarvabhuta) in all respects (sarvatha) and for all times (sarvada)."
So do not go around bullshitting that vegetarianism was taken from Jainism and Buddhism.
Sigh.
Are you aware that "Vyasa" is not a person, but a title? And that the notions of ahimsa evolved through the centuries, and that Buddhism and Jainism influenced these thoughts?
Vegetarianism was not "taken" from Buddhism or Jainism - it was a cultural practice that spread with the spread of those religions. That is why Rishis in the Rig Veda were meat eaters, but Brahmins today are not supposed to be.