You forget that
Vulcans are not without emotion, they just bury them under what is arguably a thin veneer of logic. To me, Vulcans lie and scheme just as much as the Romulans, but they say it is logical, as if that will cover it. Very few Vulcans appear to have achieved Kolinahr and therefore their emotions are bound to come through at times, no matter how much they bury them. She had hatred for Klingons as a girl and held on to it for decades, nurturing it and twisting her logic.
Vulcans are as susceptible as the rest of us when it comes to revenge it would seem. Just as humans become blinded by revenge, so do Vulcans, but it manifests differently, and seemingly more potently. Because they bury their emotions and don't really deal with anything, when they erupt, they do so like Pompeii instead of something more.
In my view, Valeris wasn't even sure of her own motivations at any given point, she was working on instinct, not logic.
But the problem is not with her being a vulcan but that the reasoning behind her motivation is maybe the most clichéd (just behind "I was abused as a child!") in modern fiction, her species is irrelevant in that regard, she could be a talking octopus for all the difference it makes to the use of that tired plot point.
Just because the "childhood trauma" has been used many times in fiction does NOT mean that no writer can use it ever ever ever. If plot devices were banned after a certain number of uses, then the world of fiction would be bare.
An individual reader can certainly look for the "new" and "interesting" in fiction, but seeing as this book was a tie in novel written about a universe that has had a lot of fictional exploration - expecting something new and exciting from it or any other Star Trek book every single time is IMO unreasonable. We buy these
A Vulcan's emotions do run very, very deep. They are also taught to control and suppress and have greater mental acuity and recall than humans. So a traumatic event would really impact their entire life, in a way that is deeper and more real than a human, especially if they chose to conceal the traumatic events.
Swallow's use of that plot device was necessary if he was to portray Valeris as a sympathetic character. Perhaps it would have been more "interesting" if she was a coldly calculating menace, but that also has been done in other novels.
The book was a great story overall, and one that delved into a very interesting period of time.