VANCIAN SPELLCASTING: You have I think two problems with Vancian spellcasting. Let's deal with the more general problem before dealing with the problem of divine spells.
My standing explanation for Vancian spellcasting is that no spell can manage to do something as powerful as create a ball of fire, or summon a lightning bolt, or move you from one location to another can in fact be cast in a handful of seconds, with a few words, or a simple gesture. If you want to do anything beyond the simpliest and weakest of magic (cantrips), you must in fact prepare the majority of the ritual ahead of time. The spell is then stored in a potential form within you in very much the same manner that you might store the spell in a staff, wand or anything else (such as a trap), awaiting the portion of the spell that serves as its trigger. To actually cast the spell, you simply finish the ritual, usually by proving what amounts to variables that specify the target of the invocation. This allows magic to be useful in a combat situation where otherwise it would not be.
Generally, preparing and casting a spell weakens the arcane caster in such a way that they cannot perform more rituals until after they have rested.