I usually use Don in my 5 Star spot, as Montanas lack of damage and that way i can still KO some. He also is a big psychological treat, making your opponent holding his pills back. But still my Don comes through 99% of the time...
But im just a noob, and if i had a Vickie i would definately use it over Don..
It really depends on what you're looking for, but i would go with Don. Don's massive damage is really helpfull in a clan like montana, that lacks damage. Besides Don's ability has a lower min. than the Uppers and Sakrohm bonus. How cool is that? A montana winning in no pills battles.
I prefer to use Fabio but thats only because I like cards that create a power advantage for themselves.
Also it depends what game mode you're playing in, because Don is arguably more useful in DTs where quick KOs are the aim of the game.
So basically it depends on: the rest of your deck (lacking damage), your playstyle, and your game-type