Vaulting seems to be broken in stable DayZ version. It doesn't work when an object is in front of you. Often you have to back away a lot to have it work.
Additionally when sprinting fast it does not make an adequate equivalent to the vault. It should turn into a long jump.
Hello there,
this is currently working as intended. Vaulting has been disabled when standing right in front of an obstacle to prevent accidental vaulting over edges of building or catwalks which would result in a fall to their death.
When vaulting, begin the vault a bit earlier, you should get over your obstacle easily.