I googled a bit and there are major rumors on all sides. Some say Vicki is fooled around all the time - even before Brooks. Some say she was sleeping with Brooks well before her marriage with Don ended. Some say Don foooled around all the time and that he got fired from his job at Home Depot because of touching / talking to women inappropriately. I also read that he and Jeanna had an affair!
Any conclusions we can draw would be pure speculation until Vicki and/or Don speak for themselves - which I hope they do.
somewhat related: I've decided that the OC women are the most messed up of all the cities.