When you’re trying to eliminate milk and dairy from your diet to become a vegan, the usual metaphor for quitting becomes problematic, to say the least. “Cold Tofurky,” says Jane Velez-Mitchell, gently correcting a reporter’s misstep. Cold Tofurky isn’t easy. That’s why VegUp, the support group that Velez-Mitchell founded, is gathered at Native Foods Cafe, a vegan restaurant on Connecticut Avenue, on a recent Thursday evening: to talk about why people slip up. “I’m Alka,…