@JeremyECrawford Fiendish Vigor would seem to be a game-breaker. 1st Lvl Warlock with hundreds of hps.
— Greg Brown (@PaddlerPoet) May 24, 2017
1. Warlocks get invocations at 2nd level, not 1st. 2. Temporary hit points don't stack. 3. A spell doesn't stack with itself. #DnD https://t.co/HuUStnB7re
— Jeremy Crawford (@JeremyECrawford) May 24, 2017
@rachitmodiSo as stated by your point #2, Dark One’s Blessing does NOT stack with Fiendish Vigor? That's correct.
— Jeremy Crawford (@JeremyECrawford) May 24, 2017

Is there an updated ruling for the 2024 version of Fiendish Vigor as the 2014 invocation specifically states it is cast at 1st level but the 2024 version omits that but rules that it is treated as a warlock spell and all warlock spells are cast at only one level based on the level of the Warlock.