are you doing it with a full team? everyone in correct positions?
make sure no one steps past the desk into the pre-vault room, where you plant the thermite, that voids overdrill
sometimes it takes a little bit of shifting around in your spots to work, and make absolute certain that no one steps beyond the front desk, into either of the two little hallways that lead into the vault area, as that prevents overdrill from happening.
activate overdrill first, then plant the thermite.