I'm a Viet Nam Era Vet, but I never spent a day serving outside of the USA.
There is no, N-O, valid reason for a guy to say he's a Viet Nam Vet AND NOT distinguish his service as overseals or stateside.
I don't know where Blumenthal served, except that it is clear that he did not serve in Viet Nam, or Vietnam if you prefer. Either way, for a guy to rise in Congress based on his service, then to find that his service is nothing to write home about means nothing short of, we can't trust anything he has to say on any topic.
His continued -- for four decades -- assertion that he served in Viet Nam is nothing short of a blatant lie. If he was executed tomorrow at dawn, I'd be okay with it.