I am not quite sure what in Dis oui makes me think of the entries from 30 years ago. It sounds quite contemporary for 1998 and the singer, 16 year old Mélanie Cohl is dressed very unlike the young vedettes of the 1960's. Still her young face, innocent performance and the song itself remind me of for example the Monegasque entry from 1965, the Swiss entries from both 1966 and 1967 and the Luxembourg entry from 1971, which all were sung by a newcomer artists (in French) aged just under or slightly over 20 years of age.
The young age of Mélanie Cohl (her real name being Mélanie Picron) is present in her performance on Birmingham stage in both good and bad. Her performance certainly is fresh and to some extent she seems to enjoy being on stage. On the other hand her inexperience is both shown and heard which, to me, is a little uneasy to watch.
I liked the 1998 Belgian entry very much 16 years ago, but it hasn't aged that well over the years. Despite of her 6th place in the final results, the representation in Eurovision didn't grant young mademoiselle Picron a steady career in the francophone, or even Belgian showbusiness. She made one album, sang on couple of Disney movies but then concentrated on other matters in life. According to some sources she took her chance at the French version of "The Voice" as late as in 2013, but without success.
My points 3/5.