The treasure in the vault is not a solid object, it’s a magical object which acts as a hint to alarm the World Government about what their greatest threat is going to be. Its magical properties allow it to transforms into their most imminent threat.
In other words: it transformed into a straw hat to indicate that Straw Hat Luffy will be the biggest threat to the World Government in the near future. This is deliberately shown before the attack/declaration of war by the Revolutionary Army in order to show us that their entire act of war and declaration on the World Government (which is going to be epic af) will still be less harmful than the Straw Hats.
Now, this doesn’t mean that the Straw Hats will physical arrive at the Reverie and cause destruction, what I think it means is, their impact and influence on the current Kings and other royalty, and countries they’ve helped, will be repayed and their kind nature is what’s so dangerous to the World Government.
Because Luffy has the ability of winning people over, to the point where the Celestial Dragons and World Government are going to be betrayed by many countries out of their love for the StrawHats? I think that’s why they’re a greater threat than the Revolutionary Army, and that’s why the magical object is showing a strawhat instead of the Revolutionary Army’s logo or something.
*Theory by MadeReddit2Complain