Pre-Hockenheim, it was reported the new blend supplied by Renault's works partner Total would deliver a 12 horse power boost.
It did not materialise.
"I think it's not the last word on this," German Vettel told Auto Motor und Sport, "as there are reasons why it did not work out.
"Whether we find out in a week, it's hard to say," he added, referring to the forthcoming Hungarian grand prix.
"For sure it was a disappointment, as we had hoped for more. The big step didn't work out," said Vettel, "but I hope that it is coming."
A few races ago, reigning world champion Red Bull looked to have established itself as the first force behind Mercedes.
But Vettel says Williams is now holding that position.
"Absolutely. Their pace in qualifying is outstanding," he said. "But I think we can still catch up. But since Austria they have been right at the front."
Until now, Red Bull has pointed the finger at engine supplier Renault for its lack of pace. But Vettel now suggests Milton-Keynes has some more work to do.
"We have simply lacked grip," he said. "We slide too much. The others are doing better."