This phrase "heard it on the grapevine" or "heard it though the grapevine" is a nice way of saying that one heard the information by means of gossip and rumor.
Example of use: "I heard on the grapevine that my sister-in-law got the promotion, but I really don’t know much about it."
Interesting fact about Heard It On The Grapevine
The "grapevine telegraph" was a way to express that information was received by eavesdropping on conversations going on between workers in the grapevines.
