I'd like to think that Bigfoot wouldn't have been killing things to sustain his 7'7 frame. I'd think its safe to say he wasn't going around nursing on other animals for nourishment. So with that as a baseline, if he were veg he'd be vegan. So I think, though I don't know much about him or his legend, that he/they would more or less feast on vegetation.
If Bigfoot ate animals, he'd they'd have left more of a trace, and from what I gather, he/they were fairly elusive.
Also, its how I always thought of myself as primordial, my inclination/instinct would direct me toward plant kingdom rather than the animal kingdom if I were in this world ignorant of the PR machines that dictate the conventional American diet.