Wingtip vortices are a function of aircraft wing loading (or weight), AoA (or speed) and configuration. They are greatest at high weight, high AoA. Use of flaps tends to reduce the vortex strength.
So vortices will be a problem anywhere you have large, slow aircraft, but particularly when no flap is used. Thus after take-off is a case in point.
I guess whether approach or take-off is worse, depends on difference between LW & TOW, difference between landing & take-off configuration (how much flap in each case) and difference V2 and approach speed. I would have guessed after take-off is worse though.
I couldn't comment on the 757 in particular. But in general, wingtip vortices will be worst with aircraft wings designed with low washout, low taper and a high critical AoA.
amended to better answer the question...
[ 10 October 2001: Message edited by: Oktas8 ]