The phrase 'stolen valor' is usually used to criticize two kinds of people.

1. People who basically pretend to be veterans when they never served, and people who did serve but massively lie about and exaggerate what they actually did (like pretending you were in some spec ops unit when in reality you were a chef). In instances like this, its almost always that people make books or a public presence or persona of some kind that is dependant on their false or greatly exaggerated acts of military service, though sometimes it can be as petty as some schlub getting an improper uniform and a faked veteran id to get military discounts at fucking IHOP.

2. Spouses (almost always wives) of actively serving or veterans of the military who make a huge part of their personality that 'they are the wife of a veteran!' and can often be seen being huge karens on social media, having freakouts any time they feel they are disrespected and claiming that this is unacceptable, because my husband is a soldier!, and then going on FB or TikTok or w/e and writing huge posts where they act as if all of the hardships endured by their husband are actually their own hardships. (This is made all the more ironic by the fact that many of these kinds of wives are also cheating on their military husbands)

It doesn't make any sense whatsoever for JD Vance to claim Walz is doing 'stolen valor'. Walz served, and doesn't seem to have lied about or massively exaggerated any of his activity.

He isn't pretending to be something he isn't, he is not pretending to have done things he has not done.

Stolen (falsely claimed) valor (acts of heroism, suffering for a dutiful purpose, etc.)