Spanish grammar: college 202 accents?

Well, your professor is indeed mistaken. The main purpose of written accent marks in Spanish is to indicate irregular syllabic stress. Since the word "vi" is monosyllabic (i.e. it consists of only one syllable), there is no reason for it have a written accent. Besides the irregular stress indication, written accents are also used to avoid homonyms (i.e. words otherwise spelled and pronounced the same but with different meanings). This latter purpose is why there is the word "si" meaning "if" contrasted with the word "sí" meaning "yes" (in addition to others like "que" vs. "qué"; "tu" vs. "tú"; "el" vs. "él"; "mas" vs. "más"; etc.). Anyway, even though your professor is wrong, these particular orthographic rules are quite unknown to most Spanish speakers. I am not saying that he should not know them, indeed he should (after all, he is a professor of Spanish), but still, I agree with the advice that ＊ 阿妹＊ May * gave- just bring it to his attention in a courteous way.

Some Spanish speakers write the way they speak, so that may be the case here. One of my friends always writes "Boy a la playa" instead of "voy".

You can always talk to him during office hours about it; just say that you're confused and that what's in the text book is different from what he's teaching/writing. Ask him to help you out that way (instead of just making it sound like he's wrong).