Definitely not a better healer than Lenna. Rydia only gets up to 3 stars black magic though, so even with a superior magic score Vanille should eventually outclass her. Rinoa still outclasses Vanille as a Black Mage though.
One way Vanille has an advantage over Lenna is she can use support abilities like Boost.
I see Vanille shining brightest in a caster-centric party with a SB that can lower enemy's Resistance.