Burundi's parliament has elected a successor to second Vice-President Alice Nzomukunda who quit on Tuesday.
Mrs Nzomukunda said she resigned because of corruption
Opposition MPs walked out of the vote, saying they had not been given enough information about the president's nominated candidate, Marina Barampama.
A BBC correspondent in Bujumbura says Ms Barampama is virtually unknown.
Ms Nzomukunda alleged corruption and human rights abuses as she quit - an event that threatened to overshadow a truce with the last active rebel group.
Some opposition members said Ms Barampama's election was invalid, since the walk-out of opposition parties meant that parliament lacked the required 78-member quorum for a vote.
Opposition parties had also called for the release of political leaders currently in detention in connection with an alleged coup plot, and a debate on the reasons for Ms Nzomukunda's resignation.
"All this was not done. For us, the endorsement of the new second vice-president is illegal and meaningless," Catherine Mabobori, a spokeswoman for the main Tutsi UPRONA party, told Reuters news agency.
On Thursday, the National Liberation Forces (FNL), Burundi's last rebel group, signed a ceasefire deal with the government at a ceremony in Tanzania.
The deal with is supposed to end 13 years of civil war between ethnic Tutsis and the Hutu majority.
Other former Hutu rebel groups have already laid down their arms, including those loyal to Pierre Nkurunziza, who was elected president last year.
"The signing of the agreement will hopefully silence the guns," said South African special envoy Kingsley Mamabolo at the ceremony in Dar es Salaam.
Earlier attempts at peace talks between the FNL and the government have failed.
Since independence in 1961, Burundi has been plagued by tension between the dominant Tutsi minority and the Hutu majority.
More than 300,000 people have died in the war sparked in 1993 by the assassination of Burundi's first Hutu head of state and democratically-elected president, Melchior Ndadaye.