I found Vhailor in the prison.
When I spoke to him for the first time, I kould kill him with words. I could
- say that everybody will die and when he dies, he can punish absolutely everybody that did or will exist
- convince him that there is no justice
- convince him that he is already dead.
All these options killed him and gave me exp

I let him join my group.
I talked to him, than I changed to warrior (I was a mage) in order to increase my strengh from his words.
Then I chose the same options as before, but this time I could not convince him to die, but some options made him attack me.

Is it possible to kill Vhailor with words after he joined the party?
Will I meet him in the fortress of regrets after I convinced him to die? (assuming I am evil at that point)

I do not want him to join me permanently. The most importent thing is that I want to convince Trias to seek redemption. There may be other moments where Vhailor might attack me when I act as I want to.
My Party consists of TNO, Morte, Dakkon, Annah, Grace and Nordom. I do not like Ingus and I want a full party to see lots of dialogue and for role playing reasons.

Assuming you cannot kill him with words after he joins, from a power gaming point of view it would be best to talk to him about everything the first time you look at him and then to kill him via dilouge for max exp. (unless you want him in your party, of course)

It is possible to kill him with words. There's multiple dialogue options that can cause his death. More info:
http://www.gamebanshee.com/planescapetorment/companions/vhailor.php

If the dialogue options disappeared, it's a bug. You can still kill him in a fight.

You will face Ignus if you're good character, Vhailor if you're evil. I don't know exactly when the game checks your alignment, but I believe it's when you jump inside the portal to get to the room with crystal in the center.

Maybe I found the problem.
I used the friends spell to increase my charisma when I first talked to him.
The spell ended when I hired him.

Unfortuanatly it looks like you have only the option to kill him with words when you select those topics for the first time.
I will cast friends again and see what happens.

This is not very importent. If I cannot convince him I just leave him behind.
I dislike killing people in combat if I can avoid it.
I have talked with him about everything, killing him would only give me some exp.