From what I see, E. Expert only stops negates regular miasma. It doesn't stop Vaal's "super miasma" clouds and cloak, and it won't negate his attacks or blights.
The miasma jewels do work, as does Vaal's own armor set, which has innate effluvial resistances. You won't negate the attack, but you completely negate his halving health blight. You also aren't affected by his "Drain Life" attack (or at the very least, the damage is so small, you think you're not affected). I think you take reduced damage as well, but I'm not to sure on that on.
I like wearing armor with Effluvial Expert anyway, because Vaal sometimes pushes you into pool of acid, and E.Expert reduces acid damage.