Republican National Chairman Bill Brock yesterday named Rep. Guy Vander Jagt of Michigan as the GOP convention keynoter and Sen. Nancy Landon Kassebaum of Kansas as the convention's temporary chairman. Both appointments shed some light on impending intraparty struggles.

The choice of Vander Jagt, the chairman of the Republican Congressional Campaign Committee, for the spotlighted keynoter's job was greeted coolly by backers of Rep. Robert H. Michel of Illinois, Vander Jagt's rival to succeed Rep. John J. Rhodes of Arizona as House Minority Leader.

Michel was not available for comment, but his press secretary, Mike Johnson, said the Peoria congressman, now the House GOP whip, "was given no advance word nor consulted in any way" about the publicity plum that was handed his rival.

The choice of Vander Jagt was unexpected, because former president Gerald R. Ford, who once represented a neighboring Michigan district, will be the opening-night speaker at the Detroit convention in July, and Rhodes, another member of the House hierachy, will be the convention's permanent chairman.

Brock said that Vander Jagt was "an old friend" and an "eloquent, articulate" speaker. He told reporters he planned to ask Michel "to play a very important role" at the convention, but had not approached him on the subject yet or decided what the role should be.

The choice of Kassebaum, daughter of 1936 Republican nominee Alf M. Landon, as the first woman to preside over a Republican convention session was more predictable. But it presages a fight on the Equal Rights Amendment to the Constitution, which is expected to be a major issue for Republican platform-drafters.

Asked about the subject at the press conference with Brock and Vander Jagt, ERA supporter Kassebaum said she thought "it would be important to continue the tradition [of endorsing ERA] that has been in the platform for some time."

She said she realized Ronald Reagan, the favorite for the nomination, opposes ERA, but said she was not sure how far the prospective nominee would press the platform committee or the convention to reflect his own views.

Mary Louise Smith, an Iowa national committee member and former national party chairman, told reporters that she believed removing the ERA endorsement from the platform would leave it "fatally flawed" in the eyes of many Republicans.

Brock, who confirmed reports that he expects to stay on as party chairman for the campaign period, with Reagan's blessing, sidestepped the issue by saying the platform would "express a strong commitment to equality for women and all Americans."

He said that last week in California he told Reagan he hoped the front-runner would soon initiate a "wide range of consultation" on the choice of a runningmate.

Saying that he hoped for a ticket "that has appeal to every American in every walk of life," Brock said he would not call for "a balanced ticket, because 'balance' means different things to different people."

Brock said he thought the presidential candidacy of Rep. John B. Anderson of Illinois, who is seeking ballot access as an independent after failing to win any primaries as a Republican, will "fade away" before November, because "most people do not want to throw away their vote on a protest candidate or a futile gesture."

While Brock was making the appointments, the temporary platform committee was hearing testimony on defense and foreign policy issues from a variety of witnesses.