Osasuna striker Sergio Leon cancelled out Ronaldo's 24th-minute opener in the 33rd after defender Tano had been carried off with a sickening double leg break.
Isco restored Real's lead in the 62nd and Zinedine Zidane's side added a third through Vazquez with the last kick of the game.
Real goalkeeper Keylor Navas needed to produce a string of quality saves to preserve their advantage before Vazquez sealed only a second league win in seven visits to the perennial La Liga strugglers.
Real top the standings on 49 points, leapfrogging Barcelona on 48 after the champions thrashed Alaves 6-0 on Saturday.
Zidane's side have two games in hand on their arch rivals as they bid to win the title for the first time since 2012.
(Reporting by Richard Martin; Editing by Toby Davis)
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