HITC Sport understands that Genk are looking to move the Belgian forward out on a permanent agreement.
Vossen spent last season on-loan at The Riverside Stadium, and scored eight league goals from 23 starts, although he often played in a deep-lying role.
Boro have lost leading scorer Patrick Bamford, who has returned to parent club Chelsea following a productive loan spell, and he will not be returning to the club – but Vossen could yet.
However, his parent club Genk – who are understood to be looking for over £2million from any permanent agreement – are ready to sell this summer.
But Vossen impressed a number of clubs during his time on Teesside and Boro have a number of rivals for his signature.