We do not provide a theme palette for the Vista theme, only for the newer themes (Windows8, Windows8Touch, VisualStudio2013, Office2013, Green, Office2016). Most of the controls do not have a set FontSize, so the system settings would be inherited in the controls.
So if you set FontSize of the container of the controls (Grid, StackPanel, Window) it should propagate down to the controls' texts. Where there is no FontSize property present, you could use TextElement.FontSize instead (e.g. in a StackPanel). FontSize set to the Window should do the trick the same way a FontSize property of the theme Palette would do in the newer themes.
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