I used to think Sackhoff was a good choice, but then I came to thoroughly loathe her Starbuck character... but then she was really good as Sarah Corvus on Bionic Woman. So I guess I'm agnostic about her. My mental image of Vale these days is based somewhat on Patricia Tallmann (Lyta from B5 and one of the bad guys in TNG: "Starship Mine"), but not entirely on any one person.
You know, whenever I read a book featuring Vale, I always knew there was someone in particular I imagined "as" her, but I could never put my finger on it. I now realize that it was Pat Tallman I was imagining. Thanks for clearing that up for me!