3E has various situational Advantages which give you a bonus in certain situations such as Favored Environment, which gives a +2 to either Attack or Defense when in your preferred environment (+5 with the second rank). It seems reasonable to me to allow either a +2 (for one rank) or a +5 (for two) for a Sneak Attack advantage that works against Vulnerable or Defenseless foes. A GM would be well-served to check to be sure that the player's build isn't such this applies all of the time instead of occasionally situationally.

3E has various situational Advantages which give you a bonus in certain situations such as Favored Environment, which gives a +2 to either Attack or Defense when in your preferred environment (+5 with the second rank). 

It seems reasonable to me to allow either a +2 (for one rank) or a +5 (for two) for a Sneak Attack advantage that works against Vulnerable or Defenseless foes. A GM would be well-served to check to be sure that the player's build isn't such that this applies all of the time instead of occasionally or situationally.