I never doubted Mulgrew's abilities as an actress: she obviously has a lot of arrows in her quiver. Janeway was a terrible character though, to which she contributed as much as the authors and directors. Her hissy delivery, especially in preaching mode, was very often the low point of the episode.
As for Vulcans, I found Tim Russ very convincing in his portrayal of Tuvok. As I wrote in another thread, he may not have been Oscar-worthy, but he was very good in my book. Jolene Blalock wasn't bad either, but there was the usual pressure to explore the limits of Vulcanity for dramatic purposes (in her case, to get her to tumble with an Earthling or two).