Do the rules 720.3 and 720.5 contradict each other?
There's no contradiction.

720.5 is an explanation of how to follow 720.3 in some cases. For example, if Lethal Vapors said "destroy it or sacrifice another permanent", then the creature's owner would have to sacrifice something else to end the loop. That's the kind of choice that is called for by 720.5.

In your example, neither player can be forced to activate Lethal Vapors. This is according to 720.5. The activated ability on Lethal Vapors is not called for by any object involved in the loop.

Either player however, would be able to choose to activate Lethal Vapors to end the sequence. Thus you have a situation which is legally allowed to end in a draw if both players agree, but either player could skip their next turn to keep the game going.

The key here is that 720.5 doesn't try to include "abilities of anything involved in the loop" in the scope. It just means when the mechanism of the loop inherently includes a way out - a player can't endlessly choose the other option to force a draw.