The main problem was finding a Windows Vista setup disc that fit my needs. It needed to be Home Premium, 64 Bit and German. Luckily, I found this website that provides links to Windows Vista, Windows 7 and Windows 8. After downloading and trying to burn it in my own ways, I created an ISO disk image from the Vista setup files with a programme called WBI Creator 1.2 and burned it with Windows 7's Image Burner.
After putting the CD into the tray and booting the laptop, you will see the Windows Vista setup appear. Just follow through the instructions on the screen.
When Vista was running, I needed to install some drivers - mainly the Ethernet drivers from Atheros. You can get them from the Atheros website. Most drivers can then be downloaded via Windows Update or from the websites of Acer, Nvidia or Synaptic website.
From that point on, I had no further unexpected difficulties with the rest of the installation. Still, installing and running Windows Vista x64 is a pain, since the driver support is poor. If you don't want to fight your way through all the Vista issues, maybe you should try a GNU/Linux distribution instead. Why not Debian?