Do you think, that is possible? How can I do this? Start Vista, and then type in "win.exe" on hard disc 2?
Yes, its possible.
To do this, start your computer, press the key it tells you to press at the startup screen to enter the BIOS.
Once in the BIOS, find the page that says "Boot Sequence", "Boot Priority", or something along those lines. Move the entry for the HDD containing Vista and XP up in the list until its at the top. Save the changes and exit the BIOS.
Your computer should now either boot directly into Vista or give you a boot menu where you can choose to boot into either Vista or XP, depending on whether or not the Vista BCD contains an entry for XP.
Regardless, boot into Vista, install
EasyBCD 2.0 there if its not already installed. Run it, and then use it to add an entry to the Vista BCD for W7 (with the entry pointed at the drive letter of the W7 partition as seen from Vista), and also one for XP if it doesn't already exist. For the XP entry, it'll ask you if you want to let it auto-configure. Accept the offer.
Once this is done, close EasyBCD and restart your system. Let it get to the boot menu without entering your BIOS again. It should give you a choice for all 3 systems. Select the W7 entry and press Enter to boot W7 from the Vista HDD.
Then once you're in W7, simply try the W7 backup again, and see if it works.