PHILADELPHIA — When Sen. John F. Kerry's advisors indicated recently that his service during the Vietnam War would be central to the campaign's effort to define him, they weren't exaggerating. Kerry's medal-laden service in Vietnam long has been an integral part of his political persona. But if his performance on the stump this week was any indication, references to his time as a naval officer commanding a swift boat in the Mekong Delta will be a ubiquitous part of the Democrat's presidential bid. In one 24-hour period, he invoked his service: • To fend off attacks by his Republican rivals;...