The Vietnam Veterans Memorial moved closer to groundbreaking yesterday when the National Capital Planning Commission approved "the development of a concept" for two changes in its design--placing a staff for the American flag somewhere "at the site" in Constitution Gardens and a statue of a serviceman "within the area before the apex" of the angle of the memorial's long walls of polished black granite. The same proposal will be considered by the Commission of Fine Arts on Tuesday.

Groundbreaking for the memorial, scheduled for last Monday, has been held up by Interior Secretary James G. Watt pending approval of the design changes by both commissions, each of which had previously approved the competition-winning design without the additional elements.

In its action the NCPC urged the new features "be located and designed so as not to compromise or diminish the basic design of the memorial as previously approved." The addition of the flag and the statue was a compromise solution agreed to by the Vietnam Veterans Memorial Fund, sponsors of the project, and opponents of the original design.

Precise locations and dimensions of the flag and statue have yet to be decided. Jan Scruggs, director of the fund, said yesterday that first consideration for the design of the statue would go to sculptors who proposed figurative statues as part of the design competition, which attracted more than 1,400 submissions.