What a fantastic question.
First we have to consider the physical strength of both combatants, then their special abilities (stealth and demonic magic) and finally their focus and motivation.
Firstly, strength. I don't think this needs to be discussed. Vancleef is a grade A badass who lifts weights for fun and Illidan is some nancy elf who couldn't even get a girlfriend. Source:
http://postimg.org/image/tj3tc0j1t/full/
Secondly, special abilities. As we all know magic is useless if it is being wielded by a pompous fool, also useless if you can't see the target. Vancleef would easily decimate Illidan with his mighty strength and subtle assault. Knifing him in the neck most likely. Source:
http://postimg.org/image/9mj0u0uyr/
Finally, motivation and focus. Illidan is usually pretty focused, after all he spent 10,000 years focusing on how he didn't get the girl he wanted - like an angst ridden teen. However it is well known that ninja's posses ultimate focus and compared to that all other focus is the equivalent of drooling all over yourself and falling over. Source:
http://postimg.org/image/cyjt4hk8x/
To conclude, Vancleef would safely win a fight against Illidan. Except.... Source:
http://postimg.org/image/52lnng4k7/
Thanks for reading - and thanks OP for posing such a brilliant question.