I gather you can't boot the Vista DVD at all to access any of its functions such as repair or fresh install.
One of the possible reasons for this is software corruption on the hard disk.
With most bootable CDs, such as Linux and utilities like the bootable CD for memtest86+, it's very simple: You set CD before HD in the boot order in BIOS and the CD always boots.
The XP and Vista installer CDs behave differently. The BIOS starts the CD like it normally does. The code at the start of the CD shows a message "Press any key to boot CD/DVD..." (that message comes from the CD, not from the BIOS). If you don't press a key, that starting program from the CD boots your HD instead. If you do press a key, it continues into the programs on the DVD, which starts with the Language settings page.
During those first few seconds when it shows that "Press any key..." message, the CD's program also reads several files from the HD to see what kind of Windows is installed there and check whether you are halfway through installing Windows. Certain kinds of corruption on the HD can cause the CD's starting program to lock up.
The solution is to wipe the HD (overwrite it with 0's or a random pattern). You can wipe either the whole disk or at least the first couple of tracks, which cleans the MBR, partition table, and the beginning of the first partition. Then the Vista DVD will start properly. The Windows installer will think it's a brand new HD with no partitions on it, so it will offer to make a new partition and then install Windows.
You can clean the HD with Dban (runs from a bootable CD or floppy) or Seagate Seatools for DOS. If you're going to download a program and burn it to CD, you might as well choose Seatools because Dban can only wipe the disk but Seatools can also run various hard disk tests and repair some problems, such as bad sectors.
Seagate Technology - SeaTools
If you find that the Seatools or Dban CD will not boot, then you will know it wasn't the problem I described above (corruption on the HD) because unlike the Vista DVD they just simply boot from the CD without being affected by what's written on the HD.
If that happens you will need to check other possible causes, probably faulty hardware such as the DVD drive or ram.