For Vale, I always imagined Catherine Bell from JAG in the roll. She's hot, she can act, and she can kick ass. (And she looks good in underwear, according to my desktop wallpaper.)
As for Vaughn, I know some might consider this sacrilege since the movie is subpar but Anthony Zerbe who played Dougherty in Insurrection I always imagined in the role. I just started reading the DS9 relaunch but when Nog asks why Shar isn't on the Defiant in the Gateways novel and Vaughn says, "You may not ask" or something to that effect, I just pictured Zerbe saying those lines.