If, as the saying goes, half a loaf is better than none, Harvard Square denizens may find two loaves better still when Au Bon Pain Corporation opens a second restaurant this year at 1150 Mass Ave.

The convenience store bakery chain, which already operates a restaurant at the heart of the Square in Holyoke Center, will open a 2000-sq.-ft. Au Bon Pain Express unit about five blocks east in a space formerly occupied by the Vie de France cafe, a top executive said.

Louis I. Kane, vice president of operations, said the smaller, self-service unit will seat 40 to 50 people. Kane said he doesn't foresee the new Au Bon Pain cutting in on the profits of the Square restaurant. The location is "moving towards" Central Square, Kane said.

"People who work there don't want to walk all the way to the center of Harvard Square...[They are] a totally different market."

Au Bon Pain plans to open the Express unit in a few months, although the lease with the building manager has not yet been signed, he said. "We expect there will be a lease with that building very soon," Kane said.

Managers of Vie de France, which quit business last fall, said in December that building owners Dupree Associates planned to hike rents more than 50 percent to more than $30 per square foot. A neighboring lease holder in the building, the Penguin Bookshop, also quit business that month.

Kane quashed rumors that Au Bon Pain would close the Holyoke store and relocate to the east end of Harvard Square.

"It's not true," Kane said, "We have a long term lease with Harvard University which expires in 1997. We're not planning to leave. We're happy with Harvard and they're happy with us."

Two factors may have triggered the rumors, Kane said. In addition to the planned Express unit opening, the company had considered closing a unit at University Place.

Au Bon Pain, which has annual earnings of $80 million, was started in 1977 by Pavailler, a French baking equipment manufacturer. The Au Bon Pain unit in Harvard Square opened in January 1983.

That unit seats about 60 people indoors with just over $3.5 million in annual sales. The average check is $2.50 a person. The unit serves approximately 25,000 people every week, earning $60,000. In the summer, with open air seating, the unit earns up to $70,000.