@Pesala thank you Pesala
I didn't delete the closed tabs list manually but after I typed "vivaldi://restart" in address bar, the 'Session only' seems to work.
Maybe the 'Startup with' setting doesn't matter?
@pafflick The possibility to easily create a new session with no shared cookie with any other would be definately great! It was the only nice feature of Internet Explorer and really handy both as a developer and power user.