For the damage part I totally agreee with you since this build gives a solid 2200 damage at lv20 with the WWE combo + 1s stun + 2.5s of blind, but 1-subtley is totally useless against valeera counters (tassadar, malfurion, cho gall, murky an so on), also stealth doesn't make you completely invisible: there is a weak air flickering in your spot that good gamers will never miss (if you run around too much trying to get yourself a good position). In my opinion the first pick should always be Vigor because Valeera always roams between the lanes in the first stages of the game and pesonally I can finish the mission always before lv5 and with 120 energy you can keep sinister strike as escape allowing otherwise very difficoult assaults. Another downside of subtley is that you have to succesfully land the first ability while stealthed to trigger the effect, so random AoEs become literally a nightmare.
The other talent i would never pick is smoke bomb for 3 reasons: 1-too long cooldown (60 seconds are you kidding me blizzard! How is valeera supposed to be useful once the opponents reach 10?!), this ability CD totally prevents you from approaching lone targets if they have full hp (they will simply ult you and run away or they will lay a trap, ult and kill you), also in teamfights you can strike only 1 time making your team fight 4 vs 5 for most of the game;
2-the effect is too short since it lasts only 5 seconds. Playing Valeera speed is of essence you can spend at most 0,6sec for WWE or EWE and other 0,6-1sec for 3-4 basic attacks relying on slice and dice effect, assuming you don't get hit by some CC in the meantime(stun, slow, poison etc...) 5+1.2=6.2 seconds (Vanish cooldown is 8) so your oppnents have almost 2 sec worth of time to focus you (to make it easy you die at EVERY teamfight);
3-How the hell can blizzard list this as an heroic ability since it's totally useless?!
Concluding pick cloak of shadows: 15 sec CD (that is almost the time spent beteen an assault and another), it removes bad status (poison, frozen, slow, rooted, blind and everything else beside stunned or silenced), it gives you 75 resistance against abilities (for example a lv 20 pyroblast from kael deals 2568 of damage if powered up, if you can time cloack of shadows the damage becomes 641) so in haevy teamfights you can jump right in the middle of offensive abilities to get a low hp enemy and escape easily with little damage because this ability also provides you with the unstoppable buff.
Your build is VERY good in the noobageddon that is QM but if you want to play Draft or ranked pick vigor and cloack.