Jenny McCullough, claimed the former minister repeatedly tried to undermine her when he was chairman of the Commons’ Home Affairs Select Committee where she was a deputy clerk.
Mr Vaz quit that post in 2016 after a media report, still under investigation, that he hired male escorts and appeared to offer to buy them cocaine, during a time when his committee was investigating vice laws.
Ms McCullough, who left her post in 2011 saying she felt unprotected by managers, says Mr Vaz told her she was bad at her job because she was “not a mother” and joked she might be security threat, because she was from Northern Ireland.
One flashpoint came during a taxpayer-funded committee trip to Ukraine in 2008, when Ms McCullough raised concerns over who was buying a “large and opulent” dinner arranged with a mystery friend of his called Ivan.
She said Mr Vaz responded by launching a “tirade” in a hotel lobby, telling her she did not know how the House worked and had an attitude problem.
The issue was known to her bosses, with her annual appraisal in 2008 noting she had “excellent” relations with the committee “apart from the chairman, who chose to try to bully her. Jenny understandably and properly stood her ground, which the chairman resented”.
A representative for Mr Vaz told BBC 2’s Newsnight that the dinner was with Ukrainian MPs to discuss crime and immigration, and the hospitality did not need to be declared under Commons rules as the committee did not host the meal.
Mr Vaz also denies any bullying, said his representative: “No complaint or allegation of this nature has ever been brought to his attention.
“Our client had considered that he and Ms McCullough had previously had a good working relationship, and had always considered her to be very effective as a clerk.”
The MP particularly denied ever making light of Ms McCullough’s not being a mother, or her ethnic background.
Other clerks told Newsnight, which has uncovered a series of similar allegations about MPs’ treatment of staff, that the Leicester East MP was prone to bullying, particularly when they tried to uphold standards.
They reported problems with other taxpayer-funded overseas trips including sudden cancellations, unplanned excursions, unexplained people in meetings and mystery over who was paying for what.
In one case, around 10 men from a London curry restaurant favoured by Mr Vaz allegedly turned up at his invitation, without warning to the rest of the committee, to join it on a private plane for an official visit to Bangladesh.
Mr Vaz’s representative said: “Our client cannot recall if any British Asian restauranteurs joined the committee on the flight in question.
“Our client would not be surprised however it if it were the case, as the committee inquiry’s remit included considering the longstanding issue of facilitating entry into the United Kingdom for individuals in the catering industry.”
A House of Commons spokesman told Newsnight: “We are aware that in the past the House has not had a robust process in place to deal with instances of bullying and harassment.
“We are confident that our new Independent Complaints and Grievance Policy will mean that allegations can be dealt with effectively and sensitively.”