The vinegar is pretty powerful for salting, though it doesn't dissolve a whole lot of the DMT.
So 20ml of vinegar should be fine, the step is simply visual if done thoroughly, as the vinegar is shaken with the solvent the DMT-acetate with cloud up (takes about a minute of agitation) after that several washes of warm/hot water will slowly clear the solvent again, though it take A LOT of water to completely clear it.
There is no harm in adding more than 20ml of vinegar though, lets say 20ml per 250ml solvent would be fine.