How would I go about altering the vampire script so that you progress through the vampire stages as you feed, rather than as you don't feed? Basically, if you never fed on anybody, you'd just stay at Stage 1 vampirism, and progressing to Stage 4 would require that you feed, before fading back to Stage 1 if you didn't keep it up.

Is this a particularly complex script alteration? I've looked around a bit, but haven't yet noticed a pre-existing mod that does it.

I suggest copying the existing PlayerVampireQuestScript and renaming it to something unique.

Then you need to overhaul the script. You're trying to turn this existing one upside down. Rewrite the VampireFeed function: instead of resetting the stage in that function (as it does now), you could increment the stage (refer to OnUpdateGameTime to see how Bethesda handles it). Then rewrite OnUpdateGameTime to decrease the stage of vampirism.

"simple" and "complex" are subjective so I can't say it's one or the other