The German, who completed just 37 laps after a technical problem resulting in an unscheduled engine change, worked through both an aero and a mechanical comparison test. Unfortunately, shortly after starting a long-run test, the rain arrived and although Vettel did some running on rain tyres, it effectively put an end to the day.
The track surface is completely new at Jerez de la Frontera and this was the first time it had been used in the wet and it was therefore extremely slippery.
For the final day tomorrow, Sebastien Buemi takes over at the wheel, having tested for Toro Rosso on Wednesday.
Second quickest was Nick Heidfeld who replaces Robert Kubica at short notice, the Pole suffering a tooth infection. In the morning, the German worked on the mechanical and aerodynamic set-up of the F1.08, evaluating different configurations for the remaining races of the 2008 season. When it started to rain around lunch time the team changed its plans for the afternoon.
"As we might have some rain during the forthcoming races we took this opportunity to work on the car's wet weather set-up," said Heidfeld.
Lucas Di Grassi continued for Renault, as the French team continued with its aero programme and evaluated various updates for the R28. However, the Brazilian had completed only 36 laps before the rain arrived. With similar overcast conditions forecast tomorrow, the team will concentrate on finishing the key elements of its programme at this, the final test of the season.
"It was quite cool today," said di Grassi, "but the track conditions were good in the morning and there was definitely better grip than yesterday. I'm also getting used to the car and the more laps I do, the more confident I feel. We didn't do any running in the wet, but we were able to test a few more parts this morning that the team will be taking to the race next week."
"A slightly challenging day for us," added Christian Silk, "as we didn't get all the test results we wanted to and the day finished early due to poor weather. However, we managed to cover the main items this morning, so it's not been too bad and we've got some interesting data to go through this evening. Hopefully it will be dry tomorrow and we can get a good final day of testing in."
Pedro de la Rosa took over at McLaren, as the Woking team continued its programme of aerodynamic proving of components to take to the final four races. The Spaniard also continued the KERS programme that began during yesterday's session. Once the rain fell during the final two and half hours of the day, the team elected to remain in the garage preparing the car for the final day of running tomorrow.
Alex Wurz continued at the wheel of the Honda, spending the morning session continuing the aerodynamic programme which he began yesterday. The rain resulted in a change to the planned schedule and the Austrian therefore completed a short programme on wet weather tyres.
Today marked the return to the F1 cockpit for Takuma Sato, as Toro Rosso continues to evaluate drivers for 2009.
The Japanese driver, who last raced in F1 in this year's Spanish Grand Prix, spent the early part of the session, working on set-up to get a feel for the car, while also completing an aero test. When the rain arrived mid-afternoon, it was decided to bring the test to an end, as the team had nothing to evaluate in wet conditions and did not want to take any risks with a driver new to the car.
"At the start of the day we were collecting aero data," said Sato. "Then by the end of the morning we began timed runs, so I could really feel the car and I started to enjoy the laps.
"Unfortunately in the afternoon it started to rain so we had to stop, that was a great shame, as I was looking forward to proper running in the afternoon. Although I couldn't complete the day due to the weather conditions, I really enjoyed driving the STR3. I would like to say a big thank you to Red Bull and to the team for giving me this great opportunity."
Kamui Kobayashi returned to the cockpit of the Toyota TF108. After yesterday's work in preparation for Singapore, the Japanese youngster switched focus to the other races remaining on the 2008 calendar, working on set-up and other systems. However, he was interrupted by a hydraulics issue in the morning before the rain took hold. Timo Glock will take over at the wheel tomorrow for the last scheduled test day of the 2008 season.
"After yesterday's trouble-free day of work, today was more difficult as the rain showers took running time away from all the teams," said Kobayashi. "In addition we had a hydraulics issue in the morning so we missed quite a lot of track time overall. Still, we made good progress while we were out on the circuit and this has been a solid test for me."
"We planned to spend today working on set-up and systems for the last few races but it was a disrupted day," added Test Team Manager, Gerd Pfeiffer. "We lost 90 minutes of running in the morning with the hydraulics issue. Then at 1pm we had the surprising sight of a shower here, which interrupted proceedings further. Kamui was just going out for another run when the rain returned, washing out the rest of the day. Tomorrow Timo will take over as we aim to finish off the week in positive fashion."
Propping up the timesheets, 3.89s off Vettel's pace, was Kazuki Nakajima in the Williams.