Vortex wake application is applied to the second, ie if its a two min dep separation and the preceding a/c departs at 18:29:29 secs, the subsequent a/c may not depart until 18:31:29 secs.
That's the way it's taught in all the colleges and that's the way it should be applied in the field.
I can confirm that's what I've been taught. Recently. When the nose wheel lifts on the first a/c, the clock starts eg 18:29:29. At college we were given 'fails' if we started the phrase 'cleared for T/O' before 18:31:29.