Vegeta's power level far exceeds Omni-Man's, with planet-destroying capability being his baseline rather than his ceiling.
Vegeta in his first appearance in Dragon Ball Z could destroy planets casually. Current Vegeta with Ultra Ego is a universe-level fighter. Omni-Man helped crack a planet with the assistance of two other Viltrumites. The power scaling is not remotely comparable.
Vegeta's ki blasts alone produce more destructive force than anything Omni-Man has survived. The Saiyan prince's speed in current form makes FTL look slow โ he has fought beings that transcend time and space. Omni-Man's combat speed, while impressive, is orders of magnitude slower.
The only area where Omni-Man might hold an advantage is combat experience and ruthlessness, but Vegeta is one of the most ruthless fighters in anime history. The Saiyan prince has no qualms about killing and approaches every fight with lethal intent. His warrior pride and battle genius are comparable to Omni-Man's, but his power ceiling is incomparably higher.
Omni-Man has thousands of years of combat experience and a warrior's ruthlessness that matches Vegeta's. His Viltrumite physiology grants enormous strength and FTL speed, making him a credible threat at his power tier.
Vegeta's current power level is universal, dwarfing Omni-Man's planet-level feats by orders of magnitude. His ki blasts alone exceed Omni-Man's durability, and his combat speed makes FTL look pedestrian by comparison.
Vegeta wins decisively at any point past the Saiyan Saga. The Dragon Ball power scale simply eclipses the Invincible universe. Omni-Man is a powerful warrior, but Vegeta operates in a completely different tier.
Omni-Man also fights
Vegeta also fights