A couple serving fish and chips from a mobile van splashed two attackers with hot fat after they were threatened with a golf club.
One of the men jumped into the van in the village of Pitstone, near Aylesbury in Buckinghamshire, on Thursday night and demanded money.
He then started smashing up the inside of the vehicle, while an accomplice smashed the windows from the outside.
But the owners of the fish and chip van managed to scare them away by throwing chip fat at them.
The men drove off in a dark blue late 1980s-style Ford Escort along Yardley Avenue.
Detective Constable Jan Pascoe, of Aylesbury CID, said: "This was a vicious and unprovoked attack on this couple.
"They have been left shocked and distressed by the incident and we are keen to hear from anyone who may have any information that could help police to trace these attackers."
Police said both might have burn marks on their faces from being splashed with hot cooking fat.