Vulcan Phoenix vs Galactus Lifebringer

Vulcan as Phoenix (as seen in the What If)

VS

Galactus Lifebringer

Vulcan never faced off vs any higher level opponents like Lifebringer did.

For what it's worth, the Watchers of that universe feared he may wind up destroying it though (he had already destroyed a couple of galaxies and 1/3 of the Annihilation Wave) so he was powerful at least.

Yeah, Vulcan was quite powerful in that What If--destroyed 7 galaxies sequentially before he even acquired the full PF:
http://i.imgur.com/5vJfJ3j.png
http://i.imgur.com/TIImSj2.png
http://i.imgur.com/GUJB0T8.png
http://i.imgur.com/xpZU5kf.png
http://i.imgur.com/sRkTl4u.png
http://i.imgur.com/WC1Nohn.png

Lifebringer stomps, though. He is well above galaxy-level power.

Galactus in annhillation was to be used to blow 2 universes, LB stomps