VDA stands for Vala Data Access. Is a library to access database with a GObject API. It was originally developed inside GDA (GNOME Data Access) as an extesion, but now it is a independent project.
VDA uses GDA's providers to access better supported ones like PostgreSQL and SQLite, databases.
VDA provides an API to create new providers by implementing interfaces, making easiest than in GDA to extend existing providers or create new ones.
There is a native implementation for PostgreSQL, so no GDA is required.
VDA's API can be consulted online at: VDA pages