Centinela alerta

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to Vigilant Sentry may become lethal if the number of cards in your graveyard falls below seven.

Once you activate the ability Vigilant Sentry gives itself, it doesn’t matter what happens to the number of cards in your graveyard. That ability will still resolve even if that number drops below seven in response.