Conniving is sort of plotting or conspiring secretly, so this smile is one that acknowledges something bad and takes pleasure in it. If I had booby trapped a door so that you hurt yourself when you opened it and then I saw you walking towards the door I might have a conniving smile on my face as you reached for the knob.

A wicked smile. An outward sign of his bad intentions.

I don't understand this part of the phrase: "at the sight of Vierna's obvious pleasure." Can you tell me pls what does that mean?