It seems to me that Vietnam is a negative for both candidates.

Faced with the threat of conscription into land combat in Vietnam they both
volunteered for a job that they expected to keep them safe and comfortable.
Bush's plan worked, which is the end of the story, for everyone but the
tinfoil hat brigade. Kerry's was only partially successful, which makes
things more complex, but not much.

The big difference is that whereas Bush admits to a misspent youth and wants
to talk about what he has done recently, Kerry has made Vietnam a major part
of his campaign.

Personally I think Bush is correct and people should decided on the basis of
what the candidates have done recently.