When you chose to believe the sucubus you have to confront Ela'yes in Iorveths presence but where is he located ? Upon starting this quest he seems to just disappear from his original location.

I dont know how they have it coded to handle tracked quests on the map ingame, but for this one it cant be wright, the marker, it just bounces around randomly on the map when i move around trying to find where it is.

He can sometimes be found at the outer areas of Vergen. You have to go through the guarded gate and take the stairs up the right gate (when facing outwards from Vergen). There will be a few houses there where you might be able to find Iorveth (a lot of Scoia'tael in that house). Try meditating.

I remember at some point in time he will disappear (to get reinforcement or something) but that would be too late.

Northwest end of town, near the north gate; get there by going up the right-hand stair (looking toward the Mahakam (west) gate from the marketplace) and continuing north.

If you haven't met the requirement to accuse Ele'yas (you must get the surgical tools and find the blade fragment in the corpse), he may not be there.

I think the quest also breaks if you have gone so far as to retrieve Triss's bandana.

You fail the quest if you give the bandana to Philippa!

I usually find him leaning against a pillar during the day, opposite the entrance to the elven house where there are usually a few elves sitting around on the steps. He can be hard to spot because he is behind you as you face the doorway.