Under Vassuer, Reims have spent much of the campaign safely in mid-table but three consecutive league defeats has left them three points above the relegation zone in 16th.
A 3-1 defeat to Lille on Saturday proved Vasseur's last game in charge with assistant coach Guegan appointed for the final seven games of the campaign.
"It seemed necessary to change things," explained club president Jean-Pierre Caillot.
"The team will be under the responsibility of Olivier Guegan for the last seven matches.
"Jean-Luc Vasseur was the end of the cycle in terms of his communication with the players. This decision was made for the good of the club."
Guegan's tenure will begin with the visit of Nice to the Stade Auguste-Delaune II on Sunday.
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