How does this Interaction between Lifeline and Lethal Vapors work?
No, there will not be an infinite loop. The card will not return to the battlefield until the following turn.

Once the end step has begun, and Lifeline's delayed trigger is put on the stack, it is too late for more “beginning of the next end step” stuff to happen during that end step.


  513.2. If a permanent with an ability that triggers “at the beginning of the end step” enters the battlefield during this step, that ability won’t trigger until the next turn’s end step. Likewise, if a delayed triggered ability that triggers “at the beginning of the next end step” is created during this step, that ability won’t trigger until the next turn’s end step. In other words, the step doesn’t “back up” so those abilities can go on the stack. This rule applies only to triggered abilities; it doesn’t apply to continuous effects whose durations say “until end of turn” or “this turn.” (See rule 514, “Cleanup Step.”)


So what will happen is that every end step, the card will return and then be destroyed right after (assuming no one responds to either ability). This will repeat every turn, but only once each turn. 

If instead of Lifeline, you have something that brings the creatures back immediately (such as an undying creature with Solemnity on the battlefield), then it would be an infinite loop, and the game will end in a draw unless one player chooses to respond by breaking the loop (such as activating Lethal Vapr's other ability).