To use GRASS 5.0 vector maps with the new GRASS 5.7, you first have to convert them to the new format: this can be made with v.convert.
By running this command, inside the vector directory a new folder with the same name of the map is created to contain all related files. The two maps (the one in 5.0 format and the one in 5.7 format) can have the same name because are stored in different directories.

Synopsis: (example)

> v.convert input=oldmap output=newmap

For further informations