so could we say that the more AoA, the more vortex strength (and the more induced drag of course)
You've nailed it in one. Induced drag is proportional to the square of the coefficient of lift. As you slow you have to increase the angle of attack which increases the lift coefficient. Have a look at
Induced Drag Coefficient
If you have an interest in how things work a good starting point is NASA's index page here
Guided Tours of the BGA One would hope, pray and think that NASA may have a bit of a handle on this stuff.
