Since I'm on a 1-time-spree, here is some more Headcanon of mine
- The assasination was actually meant to be against both Vivi and Cobra
- Sabo was somehow present when Cipher Pol or Kuma or some other person was executing the deed, starting with shooting Cobra or something
- In a moment of quick wit, he realized three things: 1.) The second shot would be against Vivi and 2.) Since only he, Vivi, Cobra and CipherPol/whoever the assassin is are present at the scene, he'll probably be framed for it; and if he's being framed, then 3.) Saving Vivi would not only save the princess' life, but also provide him with an important witness to prove his innocense
- This Point 3.) will be a callback to Alabasta by Oda, since Vivi back then was also (apart from the SHs; and Cobra who was far away) the only one during the big battle between rebels who knew that Crocodile was a fraud and her father was innocent; similarly, she's not the only one that knows of Sabo's innocence
- So while Cobra got shot in front of Vivi's eyes, Sabo quickly turns towards a shocked Vivi, grabs her and flees the scene, possibly getting help from his Rev Commanders and perhaps even Kuma who would block off the path for Cipher Pol who chase after Sabo (if Kuma is not the one doing the assasinating, that is; Kuma would not be rescued as intended by Bonney and the Revs, but stays behind to protect them and allow them to flee)
- Sabo reveals in just a few words that he's Luffy's brother, and Vivi therefore trusts him and they escape Mary Joa
End of Headcanon
Admit it, that would be pretty cool, wouldn't it