Twelve veterans groups announced yesterday that they had formed a national Vietnam Veteran Coalition to lobby Congress for passage of an Agent Orange compensation bill and a full accounting of servicemen still listed as missing in action or prisoners of war in Vietnam.

Thomas Wincek, chairman of the Vietnam Veterans Foundation, said the coalition was formed to give Vietnam veterans a single voice and more political clout. It is the first organization to pull a wide variety of Vietnam veteran groups together. The group gave Harry N. Walters, head of the Veterans Administration, and White House personnel a list of eight requests yesterday. They asked the administration to support pending legislation to provide VA disability payments to Vietnam veterans suffering from three diseases possibly caused by the herbicide Agent Orange.

They also asked for diplomatic pressure for an accounting of the missing in action and prisoners of war, the appointment of Vietnam veterans to high-visibility government jobs and a jobs program specifically for Vietnam veterans. The group also criticized the Small Business Administration for allocating only $5 million of a $25 million loan program Congress created for Vietnam veterans this fiscal year.

The largest veterans' group to join the coalition was the 1.9-million member Veterans of Foreign Wars, which pledged to lobby for the group's eight goals.