Fiendish Vigor mechanics

Fiendish VigorPHB'24 p155

Prerequisite: Level 2+ Warlock

You can cast False Life on yourself without expending a spell slot. When you cast the spell with this feature, you don't roll the die for the Temporary Hit Points; you automatically get the highest number on the die.

When casting this spell at a higher level, does the spell get boosted as well? Would I gain 15 points at level 5?

Casting the spell with a higher-level spell slot increases the amount of temp HP it grants as explained in the spell description. However, the benefit from Fiendish Vigor where it maxes out the amount of temp HP instead of rolling the dice only applies when you cast the spell with this feature, not when you cast it with a spell slot.

Yes, I understand. My question is whether the pact always casts it at level 1.

If you use the invocation you get the one dice.

If you use a spell slot you get multiple dice and no auto max #.

Quote from lokaodoidao >>

Yes, I understand. My question is whether the pact always casts it at level 1.

The Invocation always casts it at level 1. When spells are cast without a spell slot, they're cast at their minimum level unless otherwise specified.