| Formalhaut |
My position has changed after thinking more about my GM's ruling. I do think it's meant to be 5d8 or 5d4.
The crux of the issue is whether the use of Up Close and Personal actually creates a sneak attack situation. The rule treats a sneak attack as a hidden strike, but it doesn't say that a rogue who is given the ability to make a hidden strike with bonus damage (such as in this situation) gets a sneak attack. The precise wording is crucial and I don't think use of this particular talent actually gives rise to a sneak attack.
If the Acrobatics check succeeds, this attack applies the vigilante's hidden strike damage as if the foe were unaware of the vigilante. Otherwise, the vigilante applies the hidden strike damage he would deal if the target were denied its Dexterity bonus to AC. Only a stalker vigilante of at least 4th level can select this talent.
This is an instruction on how to apply a damage bonus. It doesn't say that the target is actually unaware of the person using this talent, and it doesn't say that the target is denied its Dex bonus to AC. Further, the language in Stalker Talent that treats the sneak attack as a vigilante hidden strike doesn't say that a hidden strike is also treated as a sneak attack. So the rogue gets to apply a damage bonus as a 10th-level vigilante, but a close reading doesn't allow that bonus to become a sneak attack.