I own a Vitus 979; one of my favorite bikes, despite having a fleet of carbon things. It takes a 25.0 post. Attempting to install a 25.4 or larger may be possible if you have a pneumatic press capable of generating several tons of compressive force. Bike and post would be destroyed in the process. When it comes to seatposts, 0.2mm is the difference between the post sliding down with gravity, and being locked up tight.
Wait for the proper post to come, and be careful with installation. Your bike frame is irreplaceable. If you have the older 'ears' type seatpost lug, do not overcook the tension and crack the lug. If you have the newer grub screw, make sure the grub screw is greased and tight! Do NOT strip the grub screw!