Celtic are understood to have opened negotiations with Vitoria about the transfer of forward Amido Balde.
According to reports the Scottish club have sent a delegation over to Portugal to try and finalise the deal.
It is thought that Vitoria own 37% of the player's rights with Catio Balde, the player's agent, having control of the remaining 53%.
Vitoria president Julio Mendes said: "The matter with Celtic is still not closed.
"Other clubs are interested in Balde and we need to study all possible options. It's complicated."
Balde is rated at £2million and has been linked with Newcastle and Everton in the past.