head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Vampire Survivors demands more variance.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.

Optimization · Pure Optimization
An auto-attacker where picking the right weapon evolutions wins by itself.
Fallout: New Vegas leans Optimization (Pure Optimization); Vampire Survivors leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Vampire Survivors
Movement is trivial; no aim or precision required.
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
Vampire Survivors
Enemies follow fixed spawn scripts; no adaptive opponent.
Vampire Survivors leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Vampire Survivors
Level-up offerings and pickup drops are randomized.
Vampire Survivors leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Vampire Survivors
Evolution paths and weapon combinations fully solve the run.
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.