We're talking about metal and masks.
There's another way to look at masking up and creating a persona.
It's a story.
The way I have understood it, Vessel is whomever is connecting to the music. Sleep represents that which drives you, that which you feel you can't live without but in fact is slowly destroying you.
Figuring out who Leo really is, is really only part of the fun of this band. Tons of lore to decipher.