That's just... Incorrect. Impa was in LoZ, AoL, OoT, OoS, OoA, TP, SS, and ALBW. Double the amount you listed. Remakes don't count, but if they did... Impa was in OoT3d.
Vaati was the main villain in two Zelda games made by Capcom that didn't sell well. And was the puppet villain of the worst selling Zelda game of all time mad by Nintendo. Probably why he was too insignificant to make an appearance in Brawl. Heck, some people believe they've spotted Impa in the Hyrule Warrioirs trailer. She was even in the CD-i games. Our favorite LoZ lesbian sure gets around. But why I don't think she'll be added is, how could they make her distinguishable from Sheik? If somebody has a moveset, I'd love to see it. She has the importance, but I'm not sure importance is an important factor in choosing characters this time around. And we all know the character selection is important in making a successful SSB. Important, important, important.