I don't understand how Betaine HCL would be a problem and ACV not.
Chyme enters the duodemun, duodemun sends hormone signal to gall bladder, gall bladder releases alkaline bile to raise pH of acidic chyme.
Most Betaine HCl is synthesized, so ACV, organic, would be the optimal choice. Since ACV is a little less acidic than stomach acid, diluting it help to reduce the impact on teeth and tissue.
I'd first test the stomach acidity level before starting anything to increase it. I like how this site approaches it: http://www.rheumatic.org/hcl.htm