Eric Mangini could not have slept well last night - if he slept at all. Sure, the Jets' 37-20 loss to the Vikings last night before an apathetic, sparsely populated Giants Stadium crowd was merely a preseason game. Sure, two weeks still remain before their Sept. 9 regular-season opener against the Patriots. Nevertheless, progression is expected at this time of year, and the Jets, who have prided themselves in the Mangini era as a well-prepared, well-focused team that doesn't beat itself, went backward with last night's putrid performance. "It's obviously very disappointing," said an agitated Mangini, who hinted strongly that a grueling week of practice lies ahead. "If anyone has the impression that it's OK because this is preseason, they're wrong."