First i was wondering on what factor of light spells damage vampire
From SupremeMagicka.ini:
Set SMLightDivisor to 20 ; Set value to divide light spell magnitudes by to determine damage to vampires
So, by default, a magnitude 100 Light spell will do 5 damage to vampires.
and if a light spell would be more damaging than a normal destruction class spell.
With the default setting, definitely not - a magnitude 100 Destruction spell would do 100 damage vs. 5 for a magnitude 100 Light. (Ignoring the effect of elemental weaknesses/resistances, for the sake of simplicity.)
But, then, there's nothing stopping you from changing your ini to set SMLightDivisor to 1, which would make Light spells equal to a Destruction spell of the same magnitude. I've never checked to see whether values below 1 are accepted or not, but, if they are, you could even set SMLightDivisor to 0.25 and have Light spells do four damage per magnitude...
I also was wondering if you can make custom spellfire spells.
As a general rule, you can't make custom spells based on scripted effects, such as Spellfire. In theory, you might be able to get away with enabling Script Effect for spellmaking/enchanting in the CS, but script effects vary so widely in how they operate that I can't imagine the results would work very well (if they even worked at all).