Documentation - Slipstream Vista Service Pack
Select the Slipstream checkbox in the Tasks page, this will disable all other pages just to show you that it must be done before any other operation.
Go to the Slipstream page, Select the Service Pack exe with the Select button, this will start the process and after it is done the GUI will be reenabled.
Now you can Rebuild the image to have only the one you selected on Browse and it will be a lot smaller. So in the Tasks page this time select the tasks you want, do not select Slipstream if it was already done, for example select Unattended and Make ISO.
After going throught the settings of the enabled pages press the Apply button.
Choose Rebuild One, after it is done go to the ISO page and create ISO ready for testing or burning.
So in conclusion, first the Slipstream and then Apply any changes, do not Apply before the Slipstream nor be worried that the GUI is disabled while Slipstream page is enabled, you can configure other options after the Slipstream.