There's no way at 5-6m wages and zero transfer fee (so a 9-11m book value) do you let him walk. You will not find an okay striker out there at that price, muchless if Vlahovic can be 80% of what he was his first few season and replicate that under Spaletti. Also, extending him gives us the option to move on from him if we need to, even for a nominal fee, and it be net neutral at worst.
9-11m book value on a top 4 club in Italy should be the backup striker.