Vile Effigy is a duplication summon cast by the boss Baal to create a clone of himself.
Baal's clone deals the same amount of damage as the original, but is easier to kill. It may not be worth the time to even bother with it however, as it also has a duration if a player (or group of players) return to town and wait for a minute or two for it to expire, at which point you can turn around and come back to continue fighting the Lord of Destruction. Sometimes Baal will cast another Vile Effigy as soon as you return, but a Scroll of Town Portal is pretty cheap, so just open another one and try again until he stops cloning himself.
The clone is created with the same percentage of remaining life, and looks identical to the real Baal, making them difficult to tell apart, but not impossible. The easiest method to tell them apart is to look at their Life bars and their label of Demon below it. The fake Baal's Demon is perfectly in line with his name, whereas the real Baal's Demon is offset to the right.
When Vile Effigy is cast, Baal will announce it by saying, "My brothers will not have died in vain..."