First of all. I did not discover this exploit and the probability of it being known for a while should be taken into consideration. This little trick will probably be widely known after unmercey has showed it in his latest movie.
The trick itself (I do not take credit for making this movie)
[ame=http://www.youtube.com/watch?v=4k4H1Vg6uc4]YouTube - Ambush Vanish Sap[/ame] (cant get the YT thing to work sorry)
Depending on your ping and serverlag theres a small ammount of time before your target enters combat after an ambush. In order to do this succesfully you might want to make a macro with a cast sequence and spam it very quickly.
/castsequence Ambush, Vanish, Sap
This one definately works for me after a bit of practice.
Another possibility for a 1 press macro is:
#show Vanish
/cast Ambush
/stopattack
/stopcasting
/cast Vanish
/stopcasting
/cast Sap
I havent had the time for test this one yet so correct me if I'm wrong here.
I couldnt find this on MMOwned, so if I am making a repost... well... too bad.
Have fun.