I've just done a quick installation of our products made with IA 2008 VP 1 and I've noticed that some variables are written (for some of those that have been set during the Pre-Install task). Otherwise you can use the Output Debug Information and I think you can force the output to be in the same file as the installation log.
I wrote a quick custom code action that calls CustomError.setLogDescription, .appendMessage, and .log. I have variables that I set with the text within the installer with Set Variable and then call the custom code action, which reads those variables and spits out stuff to the log.
