Steps to reproduce (Tested twice)

1- Take your main weapon out, raise the gun.
2- Press Z to go prone, stand again pressing C.
3- Press the hotbar key assigned to your main weapon to holster it.
4- Vault glitch.

*Fixed by clicking the [X] button on your hands in step (3) instead of pressing the hotbar key.
*To reset the glitch i had to draw the real weapon, holster, drop on the ground and pick it up again.