Luke didn't know whether or not Vader needed his helmet to survive; and that wasn't the point of his statement.
I don't think Luke is necessarily claiming that removing Anakin's helmet will kill Anakin.
Sure, he could have surmised as much as some of the other answers have stated (and I don't disagree with those assumptions); but I think there was a lot of emotion behind this statement as well.
Luke had just witnessed (and probably felt) his father coming back to the light side of the force and is in danger of losing him again. Luke saying
"But you'll die"
is not so much saying Luke thinks the helmet is keeping Anakin alive as it is Luke trying to do anything that might keep Anakin alive (and also probably beginning to accept the fact that he will not be able to save his father.)
Luke does not want to give up even when faced with the certain death of his father. So he offers up this weak defense to show that he is still willing to try.