Mr Vessey admitted making most of the changes identified, and said he had acted out of frustration and disappointment.
"I was simply upset and to some extent bitter at a commercial arrangement that had soured," he said.
He said the changes did not affect any current listings with the company.
The tribunal said the conduct was a marked or serious departure from the standards of an agent of good standing, and amounted to misconduct.
"Simply put, we accept the general submissions of ... the prosecution and indeed we add that the conduct of the defendant amounts to a type of commercial sabotage.
"Obviously, it is very wrong and disturbing for a salesperson to tamper with the business records of his or her employer."
The tribunal will take submissions before determining a penalty, but indicated a fine of $3000, plus compensation of $2000 and costs would be considered fair.