Even though Knives/Vash have good perception, that still doesn't mean they will be able to tell the difference between Orochimaru's clones, since, they are clone exactly like the user. Another thing is Knives is going to have to deal with several attacks coming his way, against a character very quick and not stationary.
Take a look at this fight, Sasuke has very good perception, not even skilled ninja's are able to sneak up on him and this was without the use of his sharingan. Sasuke using his sharingan was still tricked by a substitute from Orichimaru. Orichimaru is very fast and if Knives lose sight of him for even a sec Orichimaru will have that clone already set. He could go under ground to set up the jutsu, Knives wouldn't know the difference.
http://www.youtube.com/watch?v=V06qbzDpeEU
He could also do all of that while summoning. And the summon snake isn't dumb and is quick that Knives won't be able to dodge it.