Can a halfling bard with the Taunt feat use a Versatile Performance (Oratory) to demoralize an enemy via the Bluff skill?
No, you cannot use Bluff to Intimidate with Versatile Performance (Oratory)
but you could with Versatile Performance (Act, Comedy, Sing, or String)
Oratory only applies to Diplomacy and Sense motive, the other 4 perform types listed can be used with Bluff.
Taunt does not affect your Intimidate skill, it simply allows Bluff to be used in a way that duplicates one function of Intimidate.
Bards' Versatile performance allows them to use the "his bonus in that skill in place of his bonus in associated skills". You're still rolling Bluff, not Perform. You just use your Perform modifier.
In summary, by taking Taunt you gain the capacity to demoralize using Bluff with your Perform (Act, Comedy, Sing, or String) bonus.
The bard's extraordinary ability versatile performance when picked for the skill Perform (oratory) only allows the bard to substitute his Perform (oratory) skill bonus when making Diplomacy and Sense Motive skill checks, not when making Bluff skill checks. To use the Perform skill bonus instead of the Bluff skill bonus (such as when using the feat Taunt), pick instead for the category of versatile performance act, comedy, sing, or string.
(Presumably, the Bluff skill is included among the skills that can be used with Perform (string) because Perform (string) includes being able to play the lyre. Those Pathfinder guys are hilarious.)