It is fundamentally unclear what the designers intended. However, it is likely that you add only your ranks (plus class-skill bonus) in Oratory, with any other bonuses to Diplomacy you may have.
I started writing this, as a corroborative response to BobTheAverage's answer.
The crucial thing to note here is that Versatile Performance didn't change what check you're making. It changed the bonus you add from skill ranks to be, instead of the one sourced from your ranks in Diplomacy, the one sourced from your ranks in Perform (Oratory) - but that is not the same thing as making a different check entirely - and there are places where you can do this, e.g. substituting a Perception check for Survival at a -5 disadvantage.
It is also worth noting that effects that give bonuses or apply effects to Perform (Oratory) checks, not to be confused with e.g. enhancement bonii to the Perform (Oratory) skill, would not apply, here, by this same logic - you are not making a Perform check, you are making a Diplomacy check to which you add Perform (Oratory).
The other argument, I think, misunderstands how skills and skill ranks work.
Specifically: a skill bonus is a type of bonus. It is calculated from the ranks you have in a skill, and then further modified by whether the skill is a class skill or not, but it is a specific entity under the rules. Skill checks are checks to which you add a skill bonus, just as stat checks are checks to which you add an ability bonus.
And then, much like Greymantle before me, I realized that I had undermined my own argument.
Because the question that raises, is:
Does a check to which you add a modifier of type X, become a check of type X?
And the answer is that I don't know! It is, in full generality, unclear what is intended in most cases!
There is, however, the example of Twist Away.
While you are wearing light armor or no armor, if you are forced to attempt a Fortitude saving throw, you can use an immediate action to instead attempt a Reflex saving throw (at the same DC).
The language here is a very clear example of substituting a check (well, a saving throw, which is a special case thereof for most purposes) for another check. And the language of Versatile Performance is not that. It only addresses the skill ranks.
Ergo, barring some missing clarification, I would rule that checks are what they say they are, no matter what bonii you add to them, and thus that this is still a Diplomacy check to which you add anything that gives your Diplomacy checks benefits. Notably, this would mean that effects that boost Perform (Oratory) checks do not apply - only things that provide bonii to the skill ranks themselves (e.g. enhancement bonuses).
It's reasonable to rule the other way, and it won't break anything I know of, but I think it is less so. Skill ranks are a game entity, separate from the bonus they provide; the developers probably would not have done this if it did not somehow matter.
This is pretty complicated, and hinges on the question of "is a check to which you add your bonus from X, a check of type X", which there's no RAW I know of about. Therefore, it fundamentally depends on how you choose to answer that at your table, which you should be consistent about. However, because there are examples of substituting checks for other types of checks much more explicitly, I don't believe this is one.