Except in the case of "swift boat" Kerry, the smear wasn't a smear, but the truth (one of the few good things Corsi did was to write the book).
Back in 2004, the Vietnamese-American newspapers were regularly calling him "phan boi Kerry." Phan boi = traitor. No sympathy for Kerry in that community.
NB: I did my own research on phan boi Kerry back then, at least his Sa Dec "special ops" claims into Cambodia. Seemed like implausible BS to me, it's 55 miles away when Chau Doc is literally on the border.