Taking these edges will *never* gimp the original form of the spell?

Let's say I'm an Outlander Invoker and take Scalding Geyser. My Geyer's were previously hitting at 100% efficiency(Let's just assume spellcraft never failed on any cast), I now take Scalding Geyser. Now if I geyser someone that has no protection to fire whatsoever, but is not vuln to it, is it hitting for greater than 100% of the original damage because it has an added fire element of damage they aren't protected against?

If so, now when I hit an unprotected felar is it the original amount of damage + another % + vuln? Say 100+10+10=120% just for the sake of numbers being thrown around to make sense?

Conversely, what if I take Scalding geyser and have to fight a fire giant, and want to use the spell. Will he just be immune to the fire part and I'll be hitting as hard of geysers as if I never took the edge?