Kempes,
Vodafone actually operates in Turkey aswell. The main operator is Turkcell, with Avea also present. Depending on where you're heading in Turkey and the available coverage will probably dictate what network you select. It's likely your phone will roam on the Turkcell network unless you want to change to anotehr network.
Perhaps as a Vodafone customer, roaming charges might be less for you when using your phone abroad on a Vodafone network? Does the Vodafone Passport deal offer anything ? Maybe worth contacting Vodafone to find out.
Hope this helps,