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?