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?

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 #.