I'm afraid to say that normal milk does not contain lactic acid. The milk has to be curdled or fermented. Sour milk and some yogurts and cottage cheese therefore contain it. I think the person who suggested this was thinking of lactose which is present in all milk normally - this is a carbohydrate / sugar which has no benefit to the skin. Lactic acid is an alpha hydroxy acid (AHA) though, so should you find a source of it, it probably would exfoliate well. But for the easier option, just buy an exfoliator with AHA in. Milk does have the benefit of being soothing on the skin though, but I'd imagine yogurt would do the trick just as well.
Vaseline acts as a barrier to the skin, which prevents water from evaporating out - so it does not moisturise the skin itself per sa, but allows the skin to build up its natural moisture without the water being lost (whilst it is on the skin). This does mean that sebum will be trapped under it until it is removed, so does increase the risk of break outs if used excessively. Break outs can be a result of sebum building up in the sebaceous glands, as are comedones (blackheads).
I suggest that you perhaps do a little digging before trying something new if the advice has no scientific explanation. Otherwise, you could just waste your money for years.
Sources: NVQ 2 and NVQ 3 in Beauty Therapy, A Level in Chemistry, A Level in Biology
