One way you can do this is to inherit the GriddataBoundGrid and override OnKeyDown. There you can swap out the tab key for the down arrow key.
Protected Overrides Sub OnKeyDown(ByVal e As System.Windows.Forms.KeyEventArgs)
If e.KeyCode = Keys.Tab And e.Modifiers = Keys.None Then
Dim e1 As New KeyEventArgs(Keys.Down)
MyBase.OnKeyDown(e1)
Else
MyBase.OnKeyDown(e)
End If
End Sub