Race officials merely “noted” but did not penalise the Ferrari driver for being left of his painted grid ‘box’ at the start of the Chinese grand prix.
“I can understand what Seb wanted to do,” said Horner, who was Vettel’s former boss at Red Bull.
“He wanted to be on a less wet part of the track. But can you do that?
“We should know in black and white what is permitted in this area and what is not.”
For his part, German Vettel protested his innocence.
“I didn’t want to be on the line on the wet track, and there is nothing in the regulations about not doing that,” he said.