Can a Constructed Pugilist's Vicious Blades modification be affected by the Keen enchantment?

Question

From Constructed Pugilist:

Vicious Blades: Razor-sharp blades on the surface of the limb cause it to deal bludgeoning and slashing damage and increase its critical threat range to 19–20. A constructed pugilist who has the vicious spikes limb modification can’t select this modification.

Meanwhile Keen states:

This ability doubles the threat range of a weapon. Only piercing or slashing melee weapons can be keen. If you roll this special ability randomly for an inappropriate weapon, reroll. This benefit doesn’t stack with any other effects that expand the threat range of a weapon (such as the keen edge spell or the Improved Critical feat).

Since Vicious Blades is not an effect, but rather a physical modification that physically changes the weapon to be more, well... vicious, would Keen (or a similar effect like Improved Critical) work on a limb so modified, to grant a 17-20 crit range?

Answer 1

Unfortunately, no

Because of the way that Vicious Blades is written

increase its critical threat range to 19–20.

it falls exactly into what Keen (and basically every other Paizo critical modification effect) disallows.

In order for this to be allowed, the ability would have had to state that you gain a new attack that deals bludgeoning and slashing with a 19-20 critical range (and include or not the benefits relating to Brawlers' unarmed attacks as described in Constructed Limb). You're far better off with Vicious Spikes, in this case.