In a large mixing bowl, toss the cabbage with the vinegar and salt. Grate the carrots and optional onion directly into the bowl. Add the mayonnaise and toss well.
The slaw can be served immediately, but it is much better -- more melded and tender -- if allowed to stand and tossed occasionally for 30 minutes to 1 hour. For a very limp slaw, refrigerate several hours or overnight.
The law on cole slaw is cabbage and vinegar. All the rest is interpretation. Adjust the recipe to taste: Add more mayonnaise, substitute sour cream or yogurt for some (but not all) of the mayonnaise, or add a supplement of one or the other. Add diced green and/or red pepper if you have one, celery or caraway seeds if you like that kind of thing, or shredded celery for an extra fresh taste.