So Vivec claims his water-face makes it so he cannot lie. In the lessons, it's said he learned it from the Dreughs. However, the Lessons establish very clearly that the dreughs are liars, as is Vivec. Is the water-face full of crap?

In a society of liars, having a definitive method of of telling and knowing the truth might be a very usefull thing.

And it might be full of crap, too. I don't think so, though.

Maybe the Water-Face provides a much-needed purpose in a race of habitual liars, by allowing a Dreugh to be trusted (for once)?

All Vekhs are liars.
I am Vekh.

Vekh does not lie.

/god logic

The Water Face isn't of the Dreugh, but rather of the sea itself, which is too busy to lie. So while the Dreugh may have taught him it, its origins are separate from them.

Purely thematically, saying that something always tells the truth and then deciding that nope, no it doesn't is just a bad move.

I always figured it was a joke. A liar, by definition, claims to tell the truth.