I’ve written in the past how Basic D&D’s magic system isn’t truly Vancian. I’m not talking Vancian in the “Fire and Forget” manner which has become so reviled because it is so misunderstood. I’m talking Vancian in the sense that the flavors of the mechanics evoke the sense of one having to scrounge for and collect ancient lost and forgotten arcana at great cost.
In OD&D and (to a lesser degree) Holmes, Magic Users have access to all spells at levels they can cast at. There are even rules that imply they can simply go down to the spell emporium and buy a replacement spellbook at fixed cost to replace any spellbook they lost. In B/X, Magic Users only learn one spell/add one spell to their spellbook per level. In all of those cases, there’s no finding scrolls to learn new spells by adding them to your spellbook.
Shitlord: the Triggering settles on a hybrid of Holmes and AD&D that allows you to incorporate truly Vancian magic into your setting, and it arrives at the place that my own house-rules on magic were more or less heading.
S:tT uses Holmes’s Intelligence chart for chance to learn spells and minimum/maximum number of basic spells per level for its Bullshitter (Magic User) class. It specifies that “a beginning Bullshitter’s spell book contains as many of the eight basic first level spells as the newb character can know.” However, it goes on to specify that a MU would need to find and copy new spells into his spellbook. So, here is how it would work:
-At first level, the DM would select the MU’s INT guaranteed 1st level spells, then MU would roll to see how many of the other 1st level spells they know. Based on the wording, the rules imply that those spells are simply not in their spellbook, but they CAN be learned in the future if a)the MU finds the appropriate scroll, b)the MU has not reached the maximum known spells for their INT, and c)the MU succeeds on the chance to learn the spell.
-At third level, when an MU gains ability to cast 2nd level spells, instead of suddenly gaining access to a new spellbook full of all 2nd level spells that they either can or cannot cast, the MU can begin to inscribe 2nd level spells that he finds. Up to their minimum spells per level number, the MU would not have to roll to learn the new spell. Once the minimum number of spells per level has been reached, the MU would need to roll their % chance to learn the new spell.
While not flawless in its explanation (some of this is my own extrapolation), this offers a potential model, along with the necessary charts missing from Moldvay, of how to do AD&D-style Vancian magic in a Basic game.