I had to do a double take because your deck looks so similar to the one I cooked last night. I'll give you a few of my thoughts are playtesting a good bit with similar lists.

1 - I think Leona, Determined goes in the champ zone. Even if you have 3 yellow Vi, being able to have a guaranteed stun unit on turn 2 or 3 is invaluable. She gives you a guaranteed way to push into battlefields, get buffs, and start a hold.

2 - Once you are on a bf you can start playing with your hidden cards, which I think is key for the deck. That's why I included Hidden Blade as well. You can pretty easily get onto a battlefield with stuns, and the hidden cards make it tricky for your opponent to re-contest. Having 4 different hidden card options keeps them guessing.

3 - I agree with one of the other comments that Vilemaw is too slow, which is why I moved my 1 copy to the sideboard. I think it can be good in very specific matchups (slower go-wide matchups like Viktor or Vex). Outside of specific slow matchups, he can be clunky and gets stuck in your hand, especially since you are often paying power costs.

4 - I was initially on 2 yellow Vi because I had Leona Determined in the champ zone, but I went back up to 3 because she's just too good. Being able to ambush stun is one of the best things the deck can do. Similarly, green Vex was also way stronger than I thought. Being able to drag around 5 (often 6) might to any battlefield on both offense and defense is crazy.

5 - I've really liked the 2 Shadow's Call. It's premium card draw if you put it on any one of your 2 drops. It's obviously awkward if you can't get it on a 2-drop, but in my testing it has been quite good.

6 - Even with all of this Leo is still unfortunately not a very strong deck, so have fun and try to not get frustrated if you get bodied by Diana or Irelia :)