Okay. I'm not sure what you already know and what you don't, so I'll start from the beginning...
Induced drag is drag due to lift. From the moment wings start generating lift, a pressure differential exists between the top and bottom. That is, lower pressure on top, higher pressure on the bottom. Because of that, air tends to 'leak' around the tips of the wings, from the aforementioned higher-pressure bottom of the wing to the lower-pressure top of the wing, as you would expect it to. That flow creates two counter-rotating vortices at the wingtips. The vortex originating at the left tip rotates clockwise, and that on the left wing rotates anti-clockwise. Those vortices essentially push the departing airstream downward ("downwash"), which effectively tilts the lift vector back (the 'direction' of lift is always perpendicular to the departing airstream). Thus, the lift isn't only acting up, but there's a stream-wise component - by definition, drag. That is induced drag.
The energy-loss approach is just a different way of looking at it. In so many words, vortices obviously contain some lost kinetic energy and something (namely the engine[s]) has to make up for it, in a sense. In that way, it is a sort of drag. (Sorry, hard to explain here...)
So all I was trying to say was that yes, there is a correlation between vortex strength/intensity and drag, simply because a change in vortex strength means a change in the amount of downwash and thus drag.
Having said all of that, I suppose I should have just provided a link instead. So, check out http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html
. Sorry to have gone off on a bit of a tangent (and made a fool of myself in the process