A fleet of Omega destroyers with Shadow armor enhancement... might win against a few Vorlon personal transports (the Vorlon equivalent of a passenger car, probably capable of outrunning the fastest Earth ship).
However, the Vorlons would not send their equivalent of a motorcycle gang. They would send their military, and Earth Force would be totally helpless against numerous Vorlon cruisers (or even one unsupported Vorlon planetkiller).
One might suspect that about fifty Vorlon cruisers would be enough to destroy Earth. Slowly but surely, they would take care of the entire Earth fleet, and destroy the planet surface either with bombs, mass drivers, biological weapons or even extended fire from their energy weapons (nobody to stop them).
Now the Vorlon planetkiller is a totally different class. It probably exceeds the total tonnage of the entire Earth spacefleet. As far as I remember, a planetkiller was defended by hundreds of Vorlon cruisers, each accompanied by tens of fighters and personal transports. Only a small portion of that fleet would be sufficient to destroy a planet inhabited by the younger races.
Given the inefficiency of nuclear weapons in empty space, one hit from a fusion bomb would probably just shake it. Two hits would make the Vorlons annoyed. Three hits would make them angry. I would expect that to seriously hurt (temporarily incapacitate) the Vorlon planetkiller, one would need to detonate at least ten fusion bombs in close proximity.
Given its heavy defenses, that would be impossible. Why? Because otherwise, the Shadows would immediately do it. If such a planetkiller is meant to act against Shadows, and remain operational for more than one second, surely it must be very well protected.
Ideally, I think the Vorlons might wish it to withstand an attack from a Shadow planetkiller. However, given the nature of the Shadow planetkiller (especially the difficulty in attacking something very thin and large with no centre), I dare to suspect a Shadow planetkiller would be enough to intercept and destroy a Vorlon planetkiller.