The boss of France's PSA Group, Carlos Tavares, has told Business Secretary Greg Clark that members of Vauxhall's pension scheme "will be no worse off" should his firm buy the brand.
He also said labour commitments at Vauxhall's Ellesmere Port and Luton plants would be honoured.
PSA Group, which makes Peugeot and Citroen cars, wants to buy General Motors' Vauxhall and Opel businesses.
Vauxhall's 15,000-strong pension scheme is one of the UK's largest.
Mr Clark, who had a meeting with Mr Tavares on Friday, said: "The meeting was reassuring. We discussed how PSA's approach is to increase market share and expand production, rather than close plants.
"I was assured that the commitments to the plants would be honoured. There was also recognition that members of the Vauxhall pension fund will be no worse off."
Company filings to the end of 2014 - the latest available - peg the Vauxhall pension scheme with assets of about £1.8bn and liabilities of about £2.6bn, leaving a deficit of £840m.
Mr Tavares told Mr Clark that no deal has been done and that discussions continue.
"This is a very important company and workforce which has been successful and we all want it to be just as successful in the future," Mr Clark added.
Vauxhall employs 4,500 staff at plants at Ellesmere Port and Luton. About 23,000 people work in Vauxhall's retail network and 7,000 in its UK supply chain.
Unite union leader Len McCluskey, who also had a meeting with Mr Tavares on Friday, said he was optimistic.
"He [Mr Tavares] was very pro-union. He talked in terms of not being here to shut plants. That's not his nature.
"He pointed out that since being chief executive of Peugeot, he hasn't shut a single plant. So I'm going to take him on face value at the moment and hope that I am not disappointed in the future."
Unions in Germany and the UK have been concerned that a takeover of Opel and Vauxhall could lead to job losses.
Should the PSA deal go ahead, GM would leave the UK and Europe, while PSA Group would become Europe's second-largest carmaker - behind VW - supplanting Renault-Nissan.
The French state has a 14% stake in PSA, as do the Peugeot family and Chinese company Dongfeng Motor.