The MLS club have acquired the 28-year old centre-back on a season-long stint from the Portuguese giants with the view to a permanent deal.
The arrival of Canadian-born Vitoria, who has previously spent time with FC Porto and Estoril, will provide the Union with plenty of quality at the back according to the club's technical director Chris Albright.
"We're excited to bring Steven in and feel he is an excellent fit for the club," Albright said in a team release.
"He has the ability and experience to make an instant impact for us and is a player we're expecting big things from."
Vitoria arrives at PPL Park following the loan of Colombian defender Carlos Valdes to Uruguayan side Club Nacional Montevideo.
Philadelphia Union (@PhilaUnion) February 9, 2015
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