We are not talking about the vines of grapes. But the grapevine which means to heard a thing through unofficial channels rather than the official one’s. You must have heard someone saying “I heard it from the grapevine”.
Grapevine has its origins in the telegraph system invented by Samuel Morse for communication. It was initially used mostly by Army. This communication method required miles and miles of wire between the source and destination. These wires had to be supported on the poles erected at specific spaces. The wires hanging on resembled the wires for supporting the grape vines. And hence the word.
Grapevine is used mostly to spread rumours because source is ambiguous. People can neither reject it or accept it entirely. But grapevine spreader sow the seeds of doubt in the minds of their targets.
Today the grapevine is used as an informal channel for communication. Grapevine fires up imagination and recipients try to verify the information. So when the information from all employees in a company is collated a bigger picture emerges.
If the source of the grapevine is murkier it spreads faster. Similarly it depends upon the interest of the recipient. For example if an employee in city A hears the grapevine that company is going to open a new outlet in city B he may not be interested in the information. But if grapevine is about the layoffs in the company he will be interested and spread it further to seek the reacts of his fellow employees.