(I will say this only because I hate Vanessa [xD], but I'll be realistic:) Looking at all points, I conclude that ... not have a chance. Sooner or later, Ferb will see how these differences may end up with what he feels.
Sorry to say, I'm not a Ferbnessa supporter. Ferb is twelve(or possibly thirteen), and she is sixteen(most likely seventeen). Vanessa is goth. Ferb is smart. Their ages....their ways..... >.< it wouldn't work out!
I, for one, support FerbXEmily. Femily fan forever!