vulnerability applies to actual damage taken (after defenses).
The Game isn't formulaic in that way. As an example Magic Item Crafting for discounts and multiple abilities is more a line item accounting process than a simple algebraic formula. If you have math skills see summation.
The game is a Work of Art rather than a technical manual. A concertive effort was made to 'simplify' the math, lol, as the books are written for middle schoolers to improve sales and PR purposes (target audience complains that 'math is hard') substituting process for standard mathmatics notation & rules. Note the abyssmal lack of formulas in spells.
PF1 carries on the style from the OGL.
Stack Exchange is good for pseudo random advice but certainly not authoritative.
TBH a relatively new designer (not staff in 2009) with good math skills was involved in PF2 which is why it has a tighter design and better grasp of standard process.
