In my case, I couldn't really tell that my VANOS seals have deteriorated. Since it happens over time, you couldn't really tell anyways. The only way was to pull the VANOS off and check the movement of the pistons-or with diagnostic equipment. Mine were very sloppy and moved very easily. I took a video of it, but for some reason, the file got corrupted. But, it just moved up and down. The rattle is obvious, either you have it or you don't. If you don't have it at 100k, I wouldn't sweat it. You may never get it. If you're going to do the VCG, you should do the VANOS seals as it is very easy to do. Since you're doing most of the job to replace the VCG, you might as well take a little more time to do the seals. If you take your time, be very careful with the pistons and clean everything thoroughly, you'll be golden. Follow the instructions on the website-they're great.
After I did mine, the VANOS pistons were very snug in the unit. I could definitely tell a difference in the low end response and the transition at around 3k-ish.
One thing-DO NOT, I repeat, DO NOT bend the metal VANOS gasket like I did. You will get a leak on the front of the unit. In fact, I had replaced the VANOS gasket with a new one about 1k miles after I did the seals. When I replaced it, I checked the movement of the pistons. It was still VERY snug! I could hear the vacuum created by moving the pistons.
Also, I have heard that some of the aftermarket VCG can have small nicks in them, so inspect it carefully before putting it on, or go OEM. I would strongly suggest OEM for the VANOS gasket as it comes with a cardboard backing to prevent the bending of the gasket during shipping or being opened by an over-zealous owner...
You can tell if the VCG has been replaced if the sealant at the transitions to the half-moon areas is copper colored or appears 'messy'. When I did mine, the factory sealant was very neat and uniform looking. It is practically impossible to get it like that with your finger.
Johnny
2004 Imola Red over Black Leather. 6-MT, Navigation, Cold Weather Pkg (the wife loves them seat warmers!), 3.46 Diff, UUC Short-Shift Kit, Tranny Mounts, CDV Delete and iPod mount.