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This question / problem has been solved by Srebrnaimage
This confused me as well. You access it through a door near the merchants gate in Vergen itself. I believe you have to go up the stairs just next to the gate and go through the elves quarter.
When you climb stairs by the gate and enter abandoned part of the town, you have something looking like mine entrance, go through there and you'll exit outside again, it will be at the end of the path
The catacombs as in the mines? in Vergen Chapter 2, you need to accept the quest with the dwarfs in the tavern, the will accompany you threw the entire place, after you accept they will be waiting for you outside where if your tracking the quest you can see where to go, they then open the door.

Now i think it is either at the highest point of vergen at either the middle or top parts, failing that it may be next to the jiant mining bucket well thing. sorry i can't be more specific than that.
Go to the gate and up the stairs to your right. Turn right and continue to where Iorveth and his army is and through the tunnel. You will arrive at the lake. Walk all the way along the lake, cross it near the waterfall and you should be able to find catacombs in the forest.