When You recieve a new update in Windows Vista integrated Microsoft Update, right click on its name & click on details. This will give you an update info window on which there will be a link to open up a webpage for downloading that update normally.
Now, go to that site & download the Update, it will be in .msu format which stands for Microsoft Update. Remember you will need to have the "Windows Update" service running if you want to install this update offline.
You can save the links & download somewhere else too & bring them to your vista computers to install.
To make an slipstreamed version, there are 3 methods
1) WAIK - It is more then 800 MB & useless for home users. It is meant for deployment scenerios.
2) vLite - Other then the fact that this is beta, it is quite good actully. But do not remove the inbuilt drivers. They are 1.3 GB in size in Vista but support a lot of devices for which Vista compatible drivers will not be relesed.
3) Put the .msu updates in the Update folder of Vista DVD. This mans you will need to edit the ISO with something like UltraISO
Personally, i don't like slipstreaming any update other then the service pack. Many times there is an update which replaces an old Update like KB12345 comes out today & then KB67890 comes out next month replacing it, so will u go & burn a new DVD again? What I do is to just download .msu file & keep it in my harddisk. Once Vista is installed I just go to install these updates one by one or using QChain, at once. This is better then making a slipstream DVD of monthly updates
