Heritage Financial Corp., a Richmond-based savings and loan holding company, has terminated acquisition talks with one Northern Virginia S&L and reached agreement to buy another, smaller institution.

The Richmond firm's proposal to buy United Savings and Loan Association of Arlington, announced last month was called off after United's directors voted to seek more Heritage stock than was offered when an initial letter of intent was signed.

This week, Heritage announced an agreement to acquire Mount Vernon S&L of Alexandria to gain a foothold in the affluent Washington metropolitan market.

In a proposed exchange of stock worth about$2 million. Heritage has offered 1.25 shares of the holding company for each of 100,320 shares of Mount Vernon S&L common outstanding, and one-quarter share of Heritage for each of 99,680 warrants for the Mount Vernon S&L (which are exercisable into common prior to June 30, 1980, on a one-for-one basis).

Mt. Vernon S&L opened for business on July 1, 1977. J. William Brennan is president. The $10 million institution has one office in Alexandria.

Heritage is the first multi-S&L holding company chartered by Virginia. Its principal subsidiary is Heritage S&L, a $212 million association with 10 offices in the Richmond area and one in Winchester.

United S&L, the subject of the earlier proposal, has resources of some $70 million. Heritage had offered 1.5 shares of its stock for each United share in a $7 million deal. United directors subsequently asked for two shares of Heritage for each United share, and this was "unacceptable to Heritage Financial's board," said Heritage President C. Linwood Clements.

In an interview yesterday, Clements said he anticipates completion of the acquisition in the second quarter next year if it is approved by state and federal regulators.