I'm not entirely sure, but I think it comes down to something along the lines of "vinegar is food that has spoiled and has become unhealthy." There's something about the process of fermenting a food into vinegar that makes it unhealthy, even when it comes from red wine, or rice wine, or something else initially compliant.
Apple cider vinegar is an exception; it's neutral for O secretors, although it's still an "avoid" for O non-secretors. If you don't know your secretor status, it's recommended that you follow the secretor values, as most people are secretors. Secretor status has nothing to do with rh status (O+ or O-) but is diagnosed with a saliva test.
Ume plum vinegar is unlisted in BTD (blood type diet) but is rated on GTD (genotype diet.) Type O individuals can either be Gatherer, Hunter, or Explorer, and all 3 of those genotypes can have ume plum vinegar. Even though the word "vinegar" is in the name, it's actually a brine, not a true vinegar.
Lemon and lime juice are also good substitutes for vinegar in recipes, such as salad dressing.