Can vinegar be used instead of alcohol in keeping sorbet soft/scoopable?
Vinegar freezes at about -2C, so it will have a minimal effect on the freezing of ice cream; certainly not the major effect of alcohol.

If you want to make a sorbet smoother without adding too much extra sugar, replace some of the sugar with dextrose--it suppresses freezing a little better than sugar, and is less sweet. You might also try stabilizers like locust bean gum or guar gum.

You might also consider skipping the syrup and just dissolve the sugar directly into the juice without the extra water (although this will affect the flavor--some fruits benefit from being diluted a bit).