Sainz had staged a late charge when he bolted on the purple-marked ultrasoft tyre, working his way down to 1m 23.134s - Toro Rosso's fastest lap across the entire eight days thus far.
That was not enough to claim top spot, however, which once again belonged to Ferrari. Vettel - who ran with the Halo cockpit protection concept early on - ended the morning with a 1m 22.852s, set on the supersoft rubber.
Behind them, Mercedes's seemingly relentless accumulation of laps was finally halted when Lewis Hamilton suffered a transmission problem and stopped on track in the closing minutes of the session.
The three-time world champion nevertheless clocked 69 laps - the most of any driver - and wound up fourth overall, around half a second down on Force India's Sergio Perez. Hamilton used medium tyres for his fastest run, while Perez had the benefit of supersofts.
Daniel Ricciardo did enough to put Red Bull fifth overall ahead of McLaren's Jenson Button, who like Sainz set his best time on the ultrasoft. The Briton was 1.5s slower than Sainz, however.
Renault's Jolyon Palmer endured another interrupted session, clocking just 31 laps - the fewest of any driver bar Rio Haryanto at Manor, who emerged from the garage for one solitary installation lap right at the end following a lengthy effort to fix a technical glitch.
Palmer wound up seventh fastest, narrowly ahead of Sauber's Marcus Ericsson, with Romain Grosjean's Haas and Felipe Massa's Williams completing the top 10.
Friday's unofficial morning testing times from Barcelona
|1||Sebastian Vettel||Ferrari SF16-H||1:22.852||54|
|2||Carlos Sainz||Toro Rosso-Ferrari STR11||1:23.134||0.282||57|
|3||Sergio Perez||Force India-Mercedes VJM09||1:23.721||0.869||44|
|4||Lewis Hamilton||Mercedes F1 W07 Hybrid||1:24.133||1.281||69|
|5||Daniel Ricciardo||Red Bull Racing-TAG Heuer RB12||1:24.427||1.575||61|
|6||Jenson Button||McLaren-Honda MP4-31||1:24.714||1.862||56|
|7||Jolyon Palmer||Renault R.S.16||1:24.859||2.007||31|
|8||Marcus Ericsson||Sauber-Ferrari C35||1:25.031||2.179||57|
|9||Romain Grosjean||Haas-Ferrari VF-16||1:25.255||2.403||53|
|10||Felipe Massa||Williams-Mercedes FW38||1:25.696||2.844||63|
|11||Rio Haryanto||Manor Racing-Ferrari MRT05||-||1|