so my inlaws (who are also vegan) gave our house a box of snacks for chanukah, including a fancy bar of chocolate with both "kosher pareve certified" and "may contain milk" on the label, which made me do a double-take when i got around to eating some of it today because uh how do you be both of those things??? i did some googling and it turns out the "may contain milk" is a voluntary thing that manufacturers put for allergy reasons, and at least according to the Orthodox Union as cited on a page about avoiding dairy allergens (i tried to find the actual page on the OU site with this text but could not, but it seems consistent with what is written there), the two are compatible, because:
The trace nuts and dairy disclaimer that is now printed on many products is there to warn consumers that although there are no nuts or dairy in the ingredients of the product itself, there is a possibility of parts per millions floating in the air and ‘contaminating’ the product.
The ‘contamination’ would only affect consumers with extremely severe allergies who can detect even the most trace amounts of the substance that they are reacting to. A product that is labeled OU (and thereby certified kosher parve) is halachically (by Jewish Law) parve. The parts per million does not affect the status of a product, because parts per million are negligible and have no halachic significance.
this charms me! it's basically how i approach being vegan. the practice we do has meaning, but at some point we have real things to care about, it doesn't matter if we ingest an atom that was in milk once. (obviously it's good to have the label warning for folks who are That Fucking Allergic! but for voluntary restrictions, like, Come Onnnnnnnnn)