Lo mein is generally not a vegan-friendly food item, but you may make it at home with only plant-based ingredients. When I saw your question, I thought that you are referring to a specific brand’s food item. Then, I looked it up on Merriam Webster, whose definition itself shows that not all of lo mein’s ingredients are vegan.
 
Going by the definition, lo mein is a dish that includes noodles. Several definitions available on the internet describe mein as a noodle. It took me a while to realize that mein and lo mein are two different products. See the difference that the two alphabets, cause in the meaning. If you are a vegan and want to try it, then search for ‘how to make vegan lo mein’ on Google or the search engine available in your area.