It's a bit hard playing dumb with a guy you know can read your mind.

I thought the whole Sith dogma thing was designed to avoid all this.

Palpatine knows Vader is plotting against him (he wouldn't be doing his job if he wasn't) it's an unspoken given when someone in one of Mario Puzo's books or in one of Graves' Claudius books refers to another character as "My friend" you know one of them will have plans to kill the other.

Having Vader play dumb is too dumb for a character like Vader.