Vancian spellcasting makes perfect sense, I don't get how it's confusing.
Here's how it works:

>Casting a spell is a lengthy ritual requiring precision and concentration.
>When you cast a spell like this, you can pause it at the penultimate moment, leaving it paused incomplete.
>If you hold the paused ritual in your mind, you can then complete it by saying the final words

You're doing the bulk of the casting ahead of time, and then pausing and concentrating on the incomplete ritual, so you can snap-cast it later.
Of course, the human mind has limits. You can only memorize a few spells like this at once, since each uses up a chunk of concenration.

Of course, as you become a more powerful magician, you can keep more such spells paused like this; your concentration is a muscle you've been exercising.

This makes actual, intuitive sense in terms of the fiction. Way more than spell-points or spontaneous casting (but still with spell slots) or whatever weirdness with preparing-but-not-really 5e does.
It's not just a game-mechanic with "~memories~" plastered over as an excuse.

You'll notice that in games with freeform magic, such as Mage, this pattern also tends to emerge. Lengthy rituals to pre-cast powerful effects that are held as stored potential to be completed at a later point.

Now, spell's having LEVELS, and so spell slots also having levels? That's fucking dumb and you don't need it. A spell is a spell, and some are better than others, and you prioritise memorising the ones you expect to need INSTANTLY, since you can slow-cast the others as needed.