Try this:
add_action( 'save_post', 'vg_update_timestamp', 0, 1 );
May be this will help…
I think i made a little mistake…But i am little confused about priorities…
so try this:
add_action( 'save_post', 'vg_update_timestamp', 10, 2 );
Because, Docs says
Lower numbers correspond with earlier execution, and functions with
the same priority are executed in the order in which they were added
to the action.