I'm not sure if anyone has truly found a way to clear this alert but I recently figured it out on my 2000 MC SS. I've ignored this "Service Vehicle Soon" alert for years. I have two scanners and also had it scanned an no one could find the bcm code on it. i can't justify a Tech 2. So I just let it be.

Well as of yesterday I did some searching and came across a YouTube video where this guy explained he was having the same issue. He unlike me has a Tech2 and found the code. He also notice his night time lights were continuously on. The code he got was for the "Ambient Light Sensor".
This little jewel is right on top of the dash close to the windshield.

Now I've also wondered about my night time lights always on but thought it was due to aftermarket bulbs. So anyway I go to my local pick n pull and get this sensor. I replace the old with this new and SOB the alert cleared.

All these damn years I couldn't find this and then I finally do another search and it was one of the simplest fixes ever on this car.

So if you still have this issue might look to replace this tiny sensor.

 

my 04 ss does "service vehicile" soon right as the evening lights come on but only when the engine reaches operating temp.
must be all sorts of combinations