[11] On 1 July 2019, Vallys signed a two-year contract with newly promoted Superliga club Silkeborg, continuing his rapid rise through the Danish divisions.

[21][22] After Vallys' received threatening messages, director of sports Carsten V. Jensen, head coach Niels Frederiksen, and club captain Andreas Maxsø publicly criticised some fans' behaviour regarding the transfer.

[20][23] Vallys made his competitive debut for Brøndby on 4 September in a 2–0 league win away against AC Horsens at CASA Arena.

This recognition came after his remarkable performances throughout 2023, contributing 13 goals and five assists in 31 appearances and earning his first cap for the Denmark national team.

[27] In August 2023, Vallys received his first call-up to the Denmark national team by head coach Kasper Hjulmand, as a substitute for injured Mikkel Damsgaard, for two UEFA Euro 2024 qualifying matches against San Marino and Finland.