When this happened in 2009, the French were not fully integrated into the NATO command structure so didnt de-conflict their submarine movements. Also if Vanguard was on station she would have been doing her best to hide using any hydrographic anomolies such as ocean fronts that she could find. In some circumstances this can very much reduce sonar detection range. The French SSBN although in transit would have been looking for the same anomolies .... two semi blind submarines in the same bit of water ... BANG.