The reigning world champion was third quickest as he took over the RB9 from team-mate Mark Webber.
"I think the car's working very well. Obviously it's difficult to judge the speed, but it terms of reliability it was good and I feel pretty happy with the car so far," said Vettel.
"It's always nice to be on top but if there's one phase when it matters least, it's now.
"It's good for us now to do a lot of laps and to see whether the car's working.
"It's an evolution and I think we've done another step forward, but that's all we can say now."
He said the use of Jerez also made the times misleading. Pirelli earlier suggested that the Spanish track should be dropped from the F1 test rota unless it was resurfaced.
"It's impossible to give a judgement right now because I think people were playing with the fuel loads today," Vettel said.
"Plus it's the first test and it's Jerez, where the tyre degradation is quite big and tyres play a massive role. There are too many variables.
"Today it was nice to be back in the car and not much more than that. We shouldn't get excited in any way.
"Now is the time to do our homework. Maybe it's not the most exciting part of the season, but I had fun today, so it's not that boring."
AUTOSPORT special testing coverage:
Gallery Testing blog Technical blog Live commentary Trackside analysis