Vinatier, who works for a Swiss conflict mediation group, was detained as ties between France and Russia deteriorated.
More than 100,000 protesters marched in French cities to protest President Macron’s appointment of Michel Barnier as PM.
By placating the far-right National Rally, Macron has given it credibility despite its election loss, say analysts.
Sale comes as European nations try to coax Serbia away from traditional ally and weapons supplier Moscow.