Vegnagun vs. Sin

[Little Blue]

No, it was very much created to destroy Sin. Bevelle knew it could destroy Zanarkand on its own.

The entirety of FFX-2 says that Vegnagun > Sin.

On the contrary, Bevelle was not even aware of Sin until after Zanarkand's destruction, and would not have built Vegnagun until after that if it was built as a response to Sin.

Just look at Maechen's dialogue, it clearly shows that Vegnagun existed during the Machina War and thus could not have been built to combat Sin as it did not exist yet.

Maechen: Where to begin? About a thousand years ago, Lenne was a popular
songstress in Zanarkand. The talk of the town, you might say. In a more peaceful age, she might have lived out her years as a performer. But the times-and her talents-did not allow such a thing to be. You see, Lenne's gift for singing was matched by her genius in the summoner's arts. When then Machina War began, all summoners were sent to the front lines. Zanarkand was hopelessly outnumbered, and Lenne knew she would not return home alive. Nonetheless, she was prepared to lay down her life to protect her people. Yet there was one person who refused to let Lenne die. Yes, he would do anything to save her. He was...a young man, Lenne's lover.

Rikku: Shuyin?

Maechen: I...don't know. His name has been lost to history. Whatever his name, the youth endeavored to steal the enemy's machina weapon and save Lenne. However...

And quite possibly as X-2 dictates, Vegnagun might be able to destroy Sin, but I'm sentimental and prefer Sin...