The way Microsoft pushes updates and forces users to monitor shutdown/reboot
regardless of whether one has the time to do it is typical of their
corporate mentality.
That being said what you did should not kill your system--I do it all the
time because I cannot change my schedule to suit Microsoft.
The failed update should be ignored or resumed and you should be able to
boot to safe mode.
It appears that when you insert the Vista DVD it does not boot to the Vista
repair/installation options? This cannot be due to a failed Vista update,
much as I despise the way Microsoft does things in a non-fallible way.
Regardless of the state of Vista on your hard drive the computer should boot
to the DVD as this process has nothing to do with the status of an existing
Vista installation.
You may have experienced a co-incident hardware failure.
For example a dying power supply can cause what you describe, as you do not
not describe any BSOD errors that are more typical of memory or hard drive
failure.
If you do not know how to troubleshoot these issues pay a repair person.