I believe that maybe it is vines instead of grapevines.

"Por portas e travessas" and "heard through the grapevine" are different idiomatic expressions with slightly different meanings.

"Por portas e travessas" is used to describe the act of doing something in a non-conventional or unusual way. On the other hand, "heard through the grapevine" is used to describe or speculate about information that has been heard unofficially or not confirmed.

Although the two expressions may have in common the fact that they refer to something that is not directly known or confirmed, they are used in different contexts. "Heard through the grapevine" is usually used to refer to rumours or gossip, while "por portas e travessas" is used to describe an action that is being done in an unconventional way.

In summary, although the two expressions may have some overlap in terms of meaning, they are not completely interchangeable, and it is important to use the correct expression depending on the context in which it is used.