Well there's only one method that I'm aware of to integrate SP1 into the Vista setup files, and from what you've been saying it's the same one that you're already aware of. It's NOT slipstreaming because slipstreaming is broken for Vista RTM, but it has a functionally similar end result.
I would suspect though that a DVD created using this method, even if you did the job exhaustively and for each flavour of Vista, would leave some files in there from the RTM that might be updated in the truly slipstreamed media from Microsoft ; and that some files that may not be needed locally might not migrate from the SP1 into the integrated media, making such integrated media unsuitable for use elsewhere than on the machine where they were created, or on 100% identical machines in certain medium- or large- scale deployment scenarios (but even in such a scenario, I would personally just obtain the slipstreamed media from MS or from some reseller --- unless my existing installation media were already heavily personalised) --- also, from what I have read on other sites that really can't be linked to here as they are direct rivals and not partners of PROnet, SP1 integration is not 100% perfect using this integration method (but only 99.9% max.

).
From what you have suggested, Staff's internal testing would appear to indicate that the method is also imperfect, and given the inherent limitations of the method, this is not surprising IMO.
Anyways, it's the method you're already aware of, as it's the only method out there

--- and IMO the method should be reserved for the minority of people who have put a lot of work into creating a personalised Vista installation package, and who do not wish to lose that work.
For the average Joe, and even for the average PROnetworking geek/expert/gamer/enthusiast, I just don't think that the integration method can be recommended as a standard deployment scenario.