Initially, Hitler didn't want a true alliance with Vichy France, because he feared that Vichy would use it to regain its strength and backstab the Reich later (he was right).
As a Prime Minister in 41, Darlan was determined to push Collaboration as far as possible, as he hoped German concessions (return of POWs, end of occupation and reparations). Hitler just wanted to use Collaboration to milk and weaken France, without giving anything important in return.
Darlan was supported by Abetz. They forced Hitler's hand by a daring scheme.
France promised to give Morocco, Oran and Mauritania to Spain, in return for Spain joining the war... But only if Hitler accepted a true alliance of equals with France.
Then, Darlan offered Mussolini a peace treaty (instead of mere armistice) conceding Tunisia, Chad, Djibouti, Syria, Lebanon, Corsica, Nice and Savoy to Italy... And promised direct support on the Libyan front and against Malta.
Mussolini and Franco backed Darlan. Hitler buckled, stopping occupation of France (except the coast), returning the PoWs and stopping the reparations.
In return, France and Spain joined the war (leading to the Fall of Malta and Gibraltar, and Axis conquest of Egypt).
Why am I talking about all of that ? Because Darlan's policies had another huge impact. Darlan (knowing he couldn't defend Indochina) decided to sell it. To Japan.
We know that Japan had plans to illegally invade the south of Indochina, which might have triggered more US sanctions (asset freezing and oil embargo) and led to war. But with Japan just buying the colony, any war between Japan and USA was avoided.