Quote from Sequilonis >>

Quote from notovny >>

Honestly, I hadn't considered the Routine Reckless Attack / Armor of Agathys option.  That said, I kind of think Dark One's Blessing and Fiendish Vigor are, at best last-ditch emergency stopgaps.

 Fiendish Vigor works pretty well at keeping you alive longer when you are a raging Barbarian with a lot of damage resistances, since the amount of Temp HP effectively becomes doubled. At Warlock 12, you would be getting 17 Temp HP, which is effectively 34 with resistance to all damage types. Add on top of that a Barbarian making 3 Greatsword attacks using Reckless Attacks and Great Weapon Master, while dealing back an average of 34 cold damage back to enemies each time they hit you (25 from Armor of Agathys and 9 from Fire Shield), and you have a pretty effective minion sweeper. The build falls apart a little bit against a single monster that can deal a boatload of damage in a single hit, but even then you are still a Raging Barbarian with 3 attacks per turn.

Fiendish Vigor doesn't make False Life a Warlock Spell, nor does it make it a Prepared Spell. (Invocations that do make the spells they grant Prepared say so, such as Pact of the Tome )  As such, it's not compatible with the Pact Magic Spell Slots, and the Invocation only allows casting it at the lowest possible Spell Level (1st) It's going to be granting just 2d4 + 4 Temp HP (average 9, Max 12) regardless of Warlock level.

Edit: Ah, forgot to read the Invocation myself,  yeah. It's just 12, you don't roll. Thanks, Character77006. 

It's going to be granting just 2d4 + 4 Temp HP (average 9, Max 12) regardless of Warlock level.

From the invocation:  "...you don’t roll the die for the Temporary Hit Points; you automatically get the highest number on the die."

Actually not quite, you max out the dice so auto 12.