The trigger for a lot of stuff OTL is the Fall of France and the subsequent Japanese occupation of FIC.
If the US is in the war, one or both of those may be butterflied away. Hell, if the US is in the war, Italy may not be. Mussolini's got to be doing the math and thinking "gosh, what a lot of ships they can park in the Med alongside the RN. Mamma mia!"
However, the US will be thinking about war with Japan as well as Germany anyway, regardless of the early declaration against just the latter (maybe because of a commerce raider doing something atrocious to an American-flagged liner or something).
It's likely that the various naval appropriations from Vinson will take place after the declaration of war against Germany, with the intent of creating a navy not just to secure the Atlantic against the pipsqueak Kriegsmarine, but also to build Essexes and Montanas etc. with which to make the Pacific an American lake after giving the IJN a paddling.
This puts Japan in a significantly worse place than OTL, with key parts of their naval order of battle (the A6M, a couple of fleet carriers, etc) not really available before late November 1941 anyway, and yet perhaps with an earlier spur to the US construction deluge that made it so clear that they had to get a US political defeat out of a string of military defeats in 1942, because after that the correlation of forces would preclude Japanese victory. Also note that earlier US committment to the war will reduce shipping losses in the Atlantic and probably get USAAF contingents into either France or North Africa, which will enable greater British committments to the Far East.
As to what happens, then the AH.com meme is that Yamamoto says that the IJN has no window where they are strong enough to pull off a Southern Strategy, he gets assassinated by Young Turks screaming yamato damashii and Japan puts all its chips on black anyway.
Bigger question; if the US is in the war before July 22 1941, does Barbarossa happen?