There is debate on the matter.
Creatures who are immune to critical hits are still affected by crit-triggered effects. You can still benefit from Flaming Burst or Vorpal against a vampire, for example; even though he takes no extra damage from the critical, the special abilities still trigger.
Someone wearing Fortification armor, however, is not immune to criticals; any critical has a percentage change of being negated, which, logically, would prevent a crit-triggered effect from occurring (since the crit never, in fact, took place).
This is Core Rules.
The problem, however, comes from the occasion when the Sage stated "Fortification doesn't really negate critical hits". (Apparently "negate" doesn't mean "negate", sometimes...)
He said that Fortification gives a percentage change of making a creature immune to a given critical... so crit-triggered effects would still take place.
So it's up to you whether you think "negate" means "negate"... or something-completely-different-to-"negate".
-Hyp.