I've been playing a warpriest, built around his fire imbue (w/ tiefling, fire domain, items etc.), and noticed that whenever the t5 - wrathful weapons comes into play (+3 imbue dice, fire changes to light) I'm experiencing a massive drop in damage, because the build is for fire, not light.

Normally I'd just skip the enhancement, but it has other goodies (+3/+3, +9 spell power) which are nearly as strong as that entire line of the tree, so I feel forced to take it, and curse my luck whenever my weapon vorpals.

I am guessing this is not the play experience you've had in mind when designing the enhancement.

I would suggest you allow the higher of the two (light/fire) to be used in case of a vorpal, which would eliminate the current issue with the enhancement.