I would get a piece of wood. 2" by 2" around 4" long.
Then drill a hole down the middle the same size as your post.
Then saw the block in half down the hole. So you have two bits with half a hole. It will look like a curved channel down each one.
Next saw both of those blocks in half so you have four bits that are 2" long with a channel down each.
Discard one of the blocks.
Fit one block on the bent bit of the post with some tape. (With the post in the channel)
Then fit the other two the other side of the post. (Again with the post in the channel of each block) Leaving a gap between them, where the bend is, secure those with some tape.
Then stick it in the vice and tighten it up. It should straighten out.
But be aware it will be weak so it will probably bend again over time.