They were likely outnumbered and being injured during the whole process.

The VC also could have removed them after being dead. I think it was less about they wanted to do it and more about psychological warfare. The whole platoon will be talking about those poor dismembered men and they may be worried theirs is next.

It doesn't to me if you walk up on them sleeping. If you have 4 people it's easy peasy, 2 put their weight on their face or chest and the other two go to chopping.