Why are Vox Repeater upgrades unavailable

I checked I missed out the charge vigour during the game. At the last stage i found it selling for 250$ in the vigour vending machine so yes they do sell it. if you happen to see a vigour skills in shop and you don't have it means your are able to buy it from the shop.

No, you can't buy it in the fair, it's just there to tease you(and to give you a vendor to "hack" with Possession).

Even if you collect all of the money you find scattered around Columbia and win both the first and the second prize at the shooting games at the fair, you'll end up with about 150 $, less than half of the price of the Bucking Bronco vigor.

Also, if you think about it from a gameplay point, getting it this early would trivialize the first part of the game. Take the first Fireman you meet, for example: it's supposed to be the first "miniboss", but using Bucking Bronco on him would render him completely harmless.

UPDATE: according to this article the vigor vendor you find at the fair is the remnant of the E3 demo. At the time the Salt system wasn't implemented yet, and the vigor bottles had "charges"