Maybe I'm not remembering as well, but Kerry was a bit of a pushover on this issue, and was not as strong of a personality as Walz appears to be out of the gate.
Walz just needs to keep dunking on Trump and Vance, pointing out that Lieutenant Bonespurs never served a minute in the military and instead has trashed veterans, that his military service was six times that of Vance (24 years v. 4 years), and that he retired at 41 years of age while Vance got out at 23 serving the bare minimum.
Shit, even Vance has said that he was "lucky to escape any real fighting," and his time in the military consisted of "working in public affairs, writing about Marines and taking photographs of their work, escorting civilian news reporters and speaking about happenings on base." Link I'm sure Walz would be careful not to denigrate those in the military taking on non-combat roles, but he should have no hesitation to compare his service record to Corporal Couchfucker, and stuff him in the corner if he or Trump try that Swift Boat bullshit.