Standing in the living room of a seedy communal house surrounded by a dozen cats and scruffy radical activists was hardly what Harvey Vieth envisioned when he traded a comfortable life as a Colorado dentist for the chairmanship of a federal task force on homelessness.

Yet that was where Vieth found himself last Sunday, hours after negotiating a delicate and unusual truce between the Reagan administration and the Community for Creative Non-Violence's Mitch Snyder, then in the 51st day of a hunger strike.

"You're not a 'fed' all your life," said Vieth, who added that attending Loyola University Dental School located near a Chicago ghetto prepared him for the grittiness of CCNV's house at 1345 Euclid St. NW.

A lanky gray-haired father of five who turns 50 today, Vieth retains the penchant for bad puns and affability of the Midwestern dentist he once was. Two years ago he sold his lucrative dental practice in Colorado Springs, left behind a decade of service as a GOP loyalist and county commissioner, and came to Washington to head the Office of Community Services in the Department of Health and Human Services.

It was Vieth who presided over the dismantling of OCS, once the antipoverty agency known as the Office of Economic Opportunity. Since his arrival the agency's workforce has shrunk from 400 to 55 and its budget from $40 million to $4.3 million.

Since January, Vieth, whose annual salary is $63,000, has been the federal government's point man on the emotional and unglamorous issue of homelessness. He has eaten in soup kitchens and toured shelters and flophouses from New York to Los Angeles.

"I've seen some things that would curl your hair," he said last week. He has seen the misery of homeless people huddled on grates and living in shelter cubicles that resemble cages. In Los Angeles, he recalled, he saw an old man die in front of a shelter.

Some advocates for the homeless question his qualifications and wonder how he can reconcile his unswerving enthusiasm for Reagan, in whose 1976 and 1980 presidential campaigns he was active, with the suffering they say those policies have wrought.

"He's a nice man, but to see and smell the streets . . . and then defend the federal policies that create homelessness" is baffling, said Robert Hayes, an attorney with the New York-based National Coalition for the Homeless.

"Vieth is not the person to be criticized for what's happened to the homeless over the last four years," Hayes added. "He's in an impossible position. There is simply no mandate for him to do what he'd like to do."

Last week's agreement, some say, is especially illustrative. It was not until the administration was confronted with escalating national publicity about Snyder's hunger strike and his possible death on election eve that it responded.

"These single-gesture incidents in no way indicate this administration is going to take a responsible, caring approach," said the Rev. John Steinbruck, chairman of the Mayor's Task Force on Homelessness.

Vieth says that although the administration acted quickly, it has long been concerned with the homeless. "Two things were very critical," said Vieth. "[HHS Secretary Margaret] Heckler was concerned about the homeless and has been for a long time, and she was concerned about Mitch."

"Also [White House chief of staff] Jim Baker was getting information at the same time," he added. CCNV members say Baker's wife Susan, an advocate for the homeless, paid a daily bedside visit to Snyder during the final week of his fast. "All this stuff was coming in from the top. They gave me the authority to go ahead and do it."

"The real value of the thing was that I trusted Harvey and he trusted me," said Snyder.