Not a fan of cooked red onions. Yellow and white, good to go.
That being said, had some pickled red onions on a meat/cheese/ veggies/ appetizer today that was pretty good. Going to play with that to see if a quick pickle works or a traditional is required
Use your vacuum sealer like you would for sous vide. I watch the cooking show "Chopped" alot - in fact, it's about the only show I've watched in months - and the contestants quite frequently use the vacuum sealer to institute a quick pickling of veggies (especially red onions) in 10-15 minutes or so, and get good reviews from the judges.