What would happen if......

What do you mean? The dark soul isn't of the flame so it cant go in the lordvessel, and is not accessible in game either. If it was of the flame it would probably act like any other lord soul.

Plus its not like you can just choose what souls to put in, and I did try to give it souls with all the DLC souls, no bono.

He may have meant lore wise. I don't really know enough about the properties behind the Dark Soul and the design of the Lordvessel, so I can't really speculate at all. For all I know, 20 clowns will pop out and start churning butter with pool noodles. I suppose I can assume with some accuracy, that either
A. The Lordvessel would accept it like any other Lord Soul
B. The Lordvessel would be overfilled and burst/explode/expound much of the energy in emergency overflow or
C. The Lordvessel would be corrupted by the Dark Soul and be forever extinguished.

I think he may have a point. No matter what the dark soul always exists, the flame always fades eventually & leads back to darkness, just fuel in the fire. But fuel always burns to ash...or cinder, eventually. But the dark soul is different, humanity is most likely tiny fragments of the dark soul, & there is always an abundance of it, or at least Lautrec seems to think so...If you were to offer the dark soul to the lordvessal somehow, maybe it would link the flame, but do it in a way where it never fades, & the curse of the undead is possibly broken, as since humanity seems to never fade, it's source is probably a source of unlimited power...enough power to corrupt weak souls like Manus.

It is a vessel for the Souls of Lords, or higher beings. The Dark Soul was split into humanity. A different type of soul essentially. While humanity can feed the bonfires, the Lord's Vessel seems to need to be fed by the souls of certain beings. Thus I imagine it wouldn't work.