perhaps if you were to search the forums you would find that this is not possible yet, wait for source, maybe they will introduce it.
the name is item_nightvision i do believe, but it don't work.
one thing i suggested before was having a hostage rescued at the start which would give teh ct's enough money to buy NV, you could do this with a triggered door, or something similar.
the only other way is to use an amx mod, write a little code (not too hard) and have it equip the player with NV straight away.