It has been established in game that China launched first, and Tim Cain confirmed it a while ago. The TV series shows that Vault-Tec had plans to drop the bombs, but we don't know if they actually did.

I’m still relatively new but I’m pretty sure that the U.S. started an invasion of China so they did a hail mary and nuked the US

The Great War (2066-October 23rd, 2077) ended with a full-on nuclear arms tradeoff (AKA total atomic annihilation, except that Vegas was mostly spared thanks to Mr. House.)

Although, I'm still not sure who actually fired first, it was probably the Chinese who fired the first shots.

After the dust settled, the world as we knew it came to an end, not with a whimper or even a bang, but a massive nuclear trade-off.

Tldr: China shot first because they were losing Resource Wars. Vault-Tec may have manipulated events into happening, we don't know.

Vault Tec dropping the bombs seems to be a red herring. It was most likely china.

Vault-Tec (the greatest company ever) had plans to trick China into dropping the first bomb, but we don’t know if the plan succeeded or it was just a coincidence that China did what Vault-Tec wanted them to do.

I was pretty sure vault-tec was behind it but im placing all my dibs on china

There's never a straight answer and there valid reasons for all four options (USA, China, Vault-Tec, or Aliens). That's one of the best things about fallout is the questions they never answer and keep us guessing.

The most commonly accepted theory is that China dropped first with heavy influence from Vault-Tec.

The show never explicitly says that Vault-Tec dropped them only that they were willing to influence to make it happen

It's not a theory or left to interpretation. China dropped the first bomb that started the Great War. This is fact.

Saxhleel12 Proof? Evidence?

Opinion is not a fact

Ignorance is not fact

What is your evidence?

What you’re doing without proof is called and assumption which makes your fact invalid.