Well, Tuvok is one of the worst Vulcans around in general. He lets his emotions get the better of him a lot, as we saw throughout Voyager, but I never saw this line as something that was ‘un-Vulcan’. Then again that’s probably because I’ve always taken this scene as Tuvok ‘giving way’ a little bit. He does that occasionally on the show, sort of putting on an emotional front or putting things in emotional terms, to help soothe various characters with whatever they’re dealing with. Considering that she was about to die and was freaking out about her kid (which was directly putting her at further risk by raising her heart rate/breathing rate in a closed environment) I just assumed Tuvok sort of… manipulated the truth a bit to calm her down.