I’m preparing a starter and I got to thinking. When the starter is finished couldn’t I just decant the wort and make vinegar with it?
Is there any reason not to try this?
Can anyone tell me how I would go from starter wort to vinegar?
Reason not to try it? Nope.
Reasons it may not work? Two. I suspect alcohol may be minimal in a finished starter because of aerobic yeast activity - and acetobacter needs alcohol to create acetic acid. Also, starters don’t taste good, also because of oxygen, and i bet bad malt makes bad malt vinegar.
You’d need a vinegar mother, our just leave it open to air and pray.
You can get a vinegar mother at the store - look for raw unpasteurized vinegar and add some of that, it will kick it off.
As for the alcohol level, it will be there but be pretty low because your starter OG was low - right? 
The problem I see is that the % acid will be too low to make vinegar that will store well. Use it quickly and don’t use it for canning or anything like that.