I can confirm this too. (and so, +rep because I haven't had the guts to post it here because I felt like it was intended... like... instants are possible during Vortex, so why shouldn't Attacks be possible? -.-)
Furthermore I can add (as a Hunter) that range-DDs could possibly "always" have the chance to DPS during Vortex. I didn't fugure out what and how to do it, but each Vortex I was either able to attack him in close combat (e.g. Raptor Strike, Mongoose Bite, etc.) or to hit him with shots (e.g. Explosive Shot, Aimed Shot or even Steady Shot if my IAotH procced right before Vortex so I got him down to 1.5 Sec casting Time, which equals my GCD)
Will try to figure out if the positioning (and how) affects "where" you rotate around him. Think of two different orbits around him, one in melee-range, one (or more?) further away.