I spent some time with Google, trying to discover actual cases of suffocation in vaults but I could not, unless you count the following two special cases:
1 - In 2000 a bank employee was accidentally locked in a vault. After a short time she pulled the fire alarm inside the vault, which filled it with CO2, killing her. I was surprised a fire system would be designed in this manner.
TRAGEDY IN BANK’S VAULT Colleagues grieve for employee who suffocated
2 - In 1893, H. H. Holmes opened a hotel plus commercial space for the Chicago world’s fair (Columbian Exposition). He proceeded to capture, torture, and murder many people. In some cases they would be locked in his bank vault and left to suffocate. The book rights have been purchased for a movie.
H. H. Holmes - Wikipedia, the free encyclopedia
So, I have not yet found any cases of ordinary/accidental suffocation. Which does not diminish how dangerous a situation it could be.