If you're intentionally cooking something in beef fat to impart beef fat flavor, that's one thing. If you happen to be using the same cooking surface that might

If you're intentionally cooking something in beef fat to impart beef fat flavor, that's one thing. If you happen to be using the same cooking surface that might have some marginal level of beef fat residue, that's something entirely different. If you adopt a definition of vegetarian that excludes any product that has a microgram of animal meat in it, you rapidly run into the problem that there is literally no vegetarian food, because dead animals end up unintentionally incorporated into pretty much all products in the processing stage. There's not even vegetarian water, because (animal) rotifers are present in it. I say this all as a vegetarian who avoids fish sauce and animal-based broths, meaning effectively large swaths of Japanese and Southeast Asian cuisines are effectively off limits to me. The attempt to one-up the majority of vegetarians by demanding convoluted purity rituals for the right to claim membership in the One True Vegetarianism doesn't help anyone.