Howdy. A Vicodin ES had 750 mg of APAP in it; if your system is that sensitive, than you are reactive to the adjunct medications, i.e. the acetaminophen. You feel better because the APAP really does help the hydro do its job. As far as I can tell--no dice for me.
That is my rationale for having my meds compounded, so I get the lowest dose of APAP allowed by law (160 mg), but other than that, if its yellow and a 10/325--I can't tell the difference. I never have--I always buy generics, because I just can't tell!
But I do agree with the lab work, as a college student, I crushed up a Percocet and a generic Perc, ran them through a high-pressure liquid chromatograph, and saw the differences that my body couldn't feel.
I do like to pre-medicate with 400 mg of Tagamet (cimetidine, and yes, the store brand WILL work)and NAC (n-acetyl cyteine) about 45 minutes before the hydro--the Tagamet prevents the hydro from being metabolised so quickly and the NAC helps to "mop up" all the poisonous APAP metabolites that could otherwise harm your liver. Otherwise, I wish my system was that sensitive to meds, I could eat a handful of Darvon and nada...

Cheers,
Lolo