Joseph A. Grundfest and Edward H. Fleischman are expected to be nominated to fill two vacancies on the Securities and Exchange Commission once their background investigations are completed.

Grundfest, 33, is a senior staff economist on the president's Council of Economic Advisers. He would fill the Democratic seat vacated by Charles L. Marinaccio. Grundfest's name was passed along to SEC Chairman John S.R. Shad by William Niskanen, former acting director of the council.

Grundfest has a law degree from Stanford University and has completed all the work toward a doctorate in economics there except for his dissertation. Before joining the CEA, he practiced law with the Washington firm of Wilmer, Cutler & Pickering.

Grundfest wrote a chapter in the president's 1985 Economic Report on hostile corporate takeover attempts, arguing there should be no federal legislation to restrict them. Grundfest was on vacation yesterday and could not be reached for comment.

Fleischman, 53, is a New York securities lawyer and a partner in Gaston Snow & Ely Bartlett. He graduated from Harvard University and Columbia University Law School and is an adjunct professor of securities law at New York University. He has been a member of the American Bar Association's committee on federal regulation of securities for 20 years. He would replace Republican James Treadway.

VACATION TIME . . . Unlike some other agencies, the SEC recognizes the dog days of August. The commission now has only three of its five seats filled and no commission meetings have been scheduled until Sept. 5. Commissioner Aulana Peters, who is vacationing abroad, left word that she would be willing to conduct business by transatlantic telephone if necessary. But chairman Shad was also on vacation.

Employes who are still on the job report that they are getting a lot of work done.

FORUM . . . The SEC's fourth annual Government-Business Forum on Small Business Capital Formation will be held Sept. 12-14 at the L'Enfant Plaza Hotel here. The forum, which is required by Congress, will consider how proposed tax reforms would affect small businesses, among other things. The speakers include Harvard University professor Howard Stevenson, Robert Ragland of the National Association of Manufacturers, Robert Gough of Data Resources Inc., and Lawrence Dildine of Price Waterhouse & Co. Federal and state securities regulation will be the subject of another panel. The session's recommendations will be forwarded to the Congress.

CUTTING BACK . . . Regional SEC offices, other than those in New York and Chicago, will no longer be required to maintain public files of reports submitted by firms in those regions, according to a July 30 notice in the Federal Register. In reality, an SEC spokesman said, the reports have not been filed for some time and the notice was only intended to bring the law into conformity with practice. Most regional offices, the notice said, do not have enough staff to maintain the files. Moreover, the SEC said that there is no longer much public interest in maintaining data on only regional companies. The SEC also noted that a number of private firms supply the information to subscribers, often by computer.