Can you combo Veiled Menace Style and Deadly Sting on an unaware enemy ?

Deadly Sting :

Activation: As an Attack action, you may use one dose of a poison and make a TN 3 Martial Arts (Air) check using a readied Concealable weapon targeting one character within the weapon’s range.

Effects: If you succeed, apply the chosen poison to your weapon (see page 244). Then, the target suffers physical damage equal to the deadliness of your weapon plus your bonus successes.

Veiled Menace :

Activation: When you make a Martial Arts [Melee] or Martial Arts [Unarmed] Attack action check using a weapon in a one-handed grip, you may spend in the following way: +: If you succeed, choose one unaware or Disoriented target of your action. That target suffers a critical strike with severity equal to your weapon’s deadliness plus 1 per spent this way beyond the first.

I don't see any reason these wouldn't stack. One is an action, the other an opportunity. Using Deadly Sting does not guarantee that the target is unaware, but since you're using a Ninjitsu technique with a Concealable weapon, there's a pretty good chance the situation would justify it (Consult your GM).

Ultimately, it's just adding the chance for Deadly Sting to cause a critical strike - which it normally can't - so it's not game-breaking, and using both does require , so you need as many results as you would to get a critical strike with a normal Strike action, you've just swapped one required for a , and made the weapon count as Razor-Edged if it wasn't already.