I also was not expecting Vadic to buy the farm this episode (I'm assuming, though with her being a Changeling, perhaps she will just reform). If that was the end for her it was as exaggerated (struggling to find the right word here) as the character's portrayal.
I was surprised but glad. I had thought that Vadic would escape with Jack and the finale would deal with the TNG cast having to save Frontier Day and rescue Jack. I am glad they did not go that route. It would have been depressing and drag things on longer. This way, Vadic got a satisfying villain death and we got a positive end with our heroes winning for a change. And the finale can focus on the big threat.
I do think Vadic is really gone. We saw her body freeze solid and shatter into a million pieces in space, not to mention the Shrike was blown to bits too. Sure, anything is possible but that looked pretty final to me. And bringing Vadic back after such a death would feel forced and dumb IMO. It would undo the emotion of our heroes victory.