When a skill is not landed feet first the deduction is not counting the skill at all is just like if the gymnast had not done anything. On uneven bars and beam the mistake usually comes on the dismount and for floor any tumbling pass that is not landed feet first, is just like if the tumbling pass was not done. The mistake is among the worst that can happen to a gymnast and the penalizations will be heavy however the athlete will still get a score. On vault since there is only one skill to count if the gymnast does not land feet first she will get an automatic zero.
The Produnova vault, attempted by several gymnasts, got scored a zero on several occasions because athletes would often land on their backs, as an example we have Yamilet Peña at the 2011 World Championships Vault Final.
For the 2017-2020 quadrenium the vault has been downgraded to a 6.4 (from its previous 7 D-Value) to discourage athletes from practicing it as many gymnasts, unprepared to do the vault, were competing it just for its high value and they were literally risking their lives as a bad landing could lead to a life-changing neck injury resulting in paralysis.
However, in less life-threatening examples of zeros scored on vault we have the all-around final in London 2012 where Hannah Whelan did not land her vault feet first and was given a zero which meant she would finish last while competing at home-Olympics. While this might have been disappointing for Hannah she was not in medal contention nor did the score affect her team in any way. Quite a different story of what a zero meant to Anna Pavlova or Ekaterina Kramarenko.
On vault there are three more reasons why you can get a zero aside from not landing feet first. One is to go before the green light flashes, meaning judges are not ready to watch you vault, (Pavlova, Beijing Olympics) the second one is to stop your run to the vault but touch the vaulting table (Kramarenko, 2007 World Championships), or to stop your run and pass the vaulting table twice (Croket, 2008 Junior Europeans)
If for whatever reason a gymnast changes her mind while running towards the vault she can run pass it and get a second chance as long as she does not touch the vaulting table. However , f this happens twice she will get a zero. At the 2008 Junior European Vault Finals Julie Croket, then 13 years old, run passed the vaulting table twice for her first vault, after she was a given a zero she performed her second vault solidly however she finished last. Julie most likely would have come close to a medal, perhaps a fourth place, if she had performed her first vault however she was very young and junior competitions are meant to build a gymnast’s character so they don’t make that kind of mistakes at high profile competitions while competing as seniors.
At the recently finished 2017 Szombathely World Cup Amy Tinkler had a similar experience after running pass the vaulting table twice on her second vault. She successfully competed it in a 3rd attempt however the vault was not scored and she finished last in a vault final where she had qualified in first place. However as disappointing as this was for Amy it was just on a personal level as it happened in a low level meet with no lasting impact to her career.
Sadly Kramerenko had the unfortunate luck to change her mind while running towards the vault but not the ability to run pass it while competing at the 2007 World Championships Team Final. It was the very last event for the Russians and they had a bronze medal basically on their hands only a disaster could take it away and a disaster happened. Kramarenko who competed during the all-around final scored a 14.025 for her vault, a number that would have been high enough for Russia to win the bronze medal in the team competition.
And Russia continued with their bad luck at the Beijing Olympics a year later. Anna Pavlova, who delivered an Amanar at the vault final so she could have a chance for a medal, went for her second vault before the green light flashed, the judges weren’t ready and she scored a zero and finished last. While even with the Amanar it was unlikely Pavlova would have medalled, the zero must have mounted into the enormous disappointment Beijing was for the Russians as they had lost team bronze due to a fall-plagued performance and Pavlova herself would finish fourth on beam and fall on the Floor Exercise Final. You can see the Pavlova incident at the end of the video “Things You Never Expected To See At The Olympics”
And at the most recent Olympics in Rio another gymnast suffered the disappointment of scoring a zero on vault. During the qualification round Ana Derek changed her mind while running towards the vaulting table and could not stop so she climbed, yes climbed, the vaulting table and scored a zero. It was a personal disappointment as finishing last in the all-around qualification was not what she wanted to give her country.
Update: During the qualification round at the 2018 Asian Games Liu Jinru experienced the same fate as Pavlova when she scored a zero after going for her second vault while the red light was still on.
Second Update: During the qualification round of the 2018 World Championships in Doha, Angelina Simakova did not land feet first while performing her vault and was awarded a zero. It was extremely lucky that the format was four-up three-count if not it would have been the Ekaterina Kramarenko drama all over again.