Had this come up the other day and couldn't find a definitive answer.
A creature declares an attack action that has a multiple strike (in this case, a Forcemaster + [mwcard=FWQ04]Galvitar, Force Blade[/mwcard] using her double strike full action, but could be a [mwcard=MW1C09]Darkfenne Hydra[/mwcard]'s triple strike or whatever) against a target with a defense (a mage with a [mwcard=FWQ01]Dancing Scimitar[/mwcard] in this case, but could be a [mwcard=MW1C22]Knight of Westlock[/mwcard] or whatever). The Forcemaster reveals [mwcard=MW1E40]Vampirism[/mwcard] and [mwcard=MW1E01]Bear Strength[/mwcard] after declaring the attack but before the avoid attack step (which might be a mistake - e.g. poor choice of timing!*). The defending mage rolls the Scimitar's defense successfully. We ruled (correctly) that we skip to step 5 of the attack and carry out the second attack of the overall attack action - e.g. the defense only avoids that first attack, but the second attack would require a second defense, which wasn't present but might apply if the mage also had [mwcard=MW1Q04]Deflection Bracers[/mwcard] and/or [mwcard=MWSTX1CKQ04]Reflex Boots[/mwcard] and/or [mwcard=MW1E07]Cobra Reflexes[/mwcard], or against [mwcard=MW1C33]Sosruko, Ferret Companion[/mwcard], etc.
The part we weren't sure about, however, was does the Vampiric and Melee +2 from the revealed enchantments apply to the second attack in this case? In the Codex, Vampiric specifically says it applies only to an attack that deals damage, and only to the first attack (e.g. won't generally trigger on later attacks in a multiple attack, or to Battle Fury or Whirling Strike, etc). Is that "the first attack that deals damage" - e.g. the second attack in this case, since the first we canceled due to a successful defense? Ditto for the +2 melee? Or are they wasted due to the first attack being neutralized?
* In hindsight, it seems the Forcemaster should have waited to see if the defense roll was successful or not, then revealed the enchantments after a failed defense roll / before the first successful attack's dice roll to get those effects with the first successful (and second, in this case) attack of the attack action.