You already seem to be aware of the 'ap' text-object that represents a paragraph, similar to that there exist 2 text-objects to deal with sentences, 'is' & 'as' for inner sentence & a (or around) sentence. You should use these along with sentence motions '(', ')' for moving them around if need be.
eg.) For moving sentence two before sentence one. I would use the following key combinations in vim : `)das(P``
Explanation :
): helps you move to the beginning of the second sentence.das: deletes the sentence.(: Moves back to the beginning of the first sentence (previous sentence to be precise)P: Pastes the previously deleted sentence before the current one.