Mild spoiler
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Well, I've just got this quest : I have to
help a young fighter doing this clean-up.
But, I don't know where I could find him
(it's not in the journal). Is he in Vas ?
Spoil me please.

Okay, glad to oblige...

Vas is on a remote island west of Dagon Fel. If you ask Percius Mercius about Vas he'll mark it on your map.

1. Leave the guild and go to Khuul, then go to the docks and take a boat to Dagon Fel.

2. Exit Dagon Fel to the west and go south when the path splits.

3. At the next split go west. Continue west past a few forks in the road; you will pass Pudai Egg Mine on your right.

4. Keep going west until you get to the next fork (where the south path leads to water. Go north, then follow the path west at the next fork.

5. Waterwalk or swim west to Vas. Use your world map to guide you.

When you get there Ulyne Henim will be waiting in front of the entrance. Speak with her about fighting together and enter the cave. Good luck.

if i remember correctly, there are quite a few enemies in there....including a daedra. For a low level char., i'd suggest packing some health potions just in case

Thanks for the directions. I'll try them.

i was low level and i got my butt kicked there

You should try going inthere when you have the Giants mod installed. Even a leveled up char will have a hard time then, not to mention the char who you are supposed to help.