Vandevoordt, the youngest keeper to start a Champions League game, conceded early on when he dwelt on a backpass and Milik scored from close range.
Milik scored from Giovanni di Lorenzo’s cross and completed his treble with a penalty after Vandevoordt fouled Jose Callejon before Dries Mertens converted a chipped ‘Panenka’ penalty late on.
In the other match in the group, Liverpool beat Red Bull Salzburg 2-0 to top Group E, with Napoli advancing as runners-up.
Genk had already been eliminated and they handed a first Champions League start to Vandevoordt, who only made his senior debut on 24 September and had featured three times for the Belgian champions this season.
At 17 years and 287 days, he became the youngest keeper in the competition, beating the previous record of Benfica’s Mile Svilar, who was 18 years and 52 days old when he played against Manchester United in October 2017.
Vandevoordt conceded after only three minutes when Jhon Lucumi played it back to him, but he did not control the ball cleanly and, closed down by two Napoli players, presented it to Milik who could not miss.
Milki completed his hat-trick in only 38 minutes before Mertens’ late penalty, following Casper de Norre’s handball, sealed Napoli’s convincing win.
Youngest goalkeepers in Champions League
1. Maarten Vandevoordt (Genk) v Napoli, 10 December, 2019 – aged 17 years 287 days
2. Mile Svilar (Benfica) v Manchester United, 18 October, 2017 – 18y 52d
3. Iker Casillas (Real Madrid) v Olympiakos, 15 September, 1999 – 18y 118d
4. Nikolay Mihaylov (Levski Sofia) v Werder Bremen, 31 October, 2006 – 18y 125d
5. Igor Akinfeev (CSKA Moscow) v Porto, 14 September, 2004 – 18y 159d
6. Adrian Semper (Dinamo Zagreb) v Lyon, 14 September, 2016 – 18y 245d