Sainz will make his first F1 start in Renault colors next weekend at COTA.
Heavy rain limits running at Suzuka on Friday afternoon as just five cars post lap times.
An issue in final practice in Malaysia puts Vettel a step closer to a grid penalty that could have huge effects on the title race.
Pierre Gasly will race with car number 10 starting at Malaysian Grand Prix.
In the space of three races, Sebastian Vettel has dropped twice as far behind Lewis Hamilton as he was ahead of him.
Fernando Alonso is the next piece of the puzzle McLaren wants to get in place for 2018.
Williams working hard to avoid a repeat of its current strife next year.