Difficulty is not something that's really been discussed in the context of Vernal Edge Speedruns. This post will run down what exactly is different between difficulties in this game.

TL;DR: Easy-Hard have the same spawns, with Easy giving you 1.5x damage, Normal 1.2x and Hard 1x. Hard and Vicious have similar enemy damage, player damage, and pulse healing, but Vicious importantly has different, longer and harder arenas from Hard.

These are the exact values of each difficulty. Screenshot of code given by Noxid, one of the devs. Thank you noxid :3

global.enemy_damage_mod: how much damage Vernal takes from enemies. Higher the number = Higher the damage taken. Scales up with difficulty

global.player_damage_mod: how much damage Vernal does to enemies. Scales down with difficulty (effectively making enemies tankier)

global.enemy_passivity_mod: seemingly affects the frequency of enemies choosing to attack. Higher the number = more aggressive the enemy is.

global.pulse_heal_mod: affects how much healing Vernal gains from Pulse attacks. Might affect how much Bloom you gain as well? Lower healing with higher difficulty.

global.spawnhardmonsters: essentially a boolean to tell the game to give you the harder spawns in arenas. only relevant in Vicious.