Dominguez indicated he saw the two politicians as well Roja's son - Emmanuel, who was born three years ago to a rebel father - after being taken to a new guerrilla camp.
"I ended up in a camp where there are more prisoners of war and there are even a few politicians held such as doctor Ingrid, Mrs Clara and a child who follows them all the time," the soldier said.
The last proof of life from 45-year-old Betancourt was in August 2003, when she was seen on a video released by Farc.
Sign of life
Dominguez said in the video that the hostages are moved to different camps on an almost daily basis due to bombing attacks by the Colombian military.
The video showed seven hostages in total, it was the first time in more than four years that the families of some of the police and soldiers held hostage had news of them.
The men are among dozens kidnapped by Farc, Latin America's oldest rebel conflict.
Dominguez was among seven soldiers and police officers shown on the new video, most of whom have been held hostage for about nine years, Holman Morris, Al Jazeera's correspondent, said.
The tape comes five days after Farc claimed that 11 politicians it had held hostage for five years died in the crossfire during a military raid on a rebel camp last month.
The government denies the military attacked the camp and accuses the guerrillas of executing the 11 provincial lawmakers.
The politicians were among 56 hostages, including Betancourt and three Americans, who the rebels want to swap for Farc members held in Colombian prisons.