Induced drag isn't only from the vortices. It also comes from angle of attack...
Again, check out reply number 6 in the thread I gave a link to above (sorry to keep refering back to this, I don't really have time to go through it all again...). In it I detail exactly what causes induced drag. Wingtip vortices are the sole source.
However, AoA certainly does affect
Cdi = k X (Cl^2 % AR
) ... where Cdi is induced drag coefficient, k is a proportionality constant, Cl is, of course, lift coefficient, and AR
is aspect ratio.
Looking at that equation, you can easily see that because an increase in AoA means an increase in Cl and an increase in Cl increases Cdi (and hence induced drag itself), AoA obviously affects induced drag.
The change in induced drag due to AoA is still because of vortices. As MITaero said, increased AoA just increases lift, which in turn amplifies vortices. That is, the vorticies are still the sole cause of induced drag.
Hope that's clear enough,