If they do a serum testing, they can differentiate between Lortab and Vicoden because of the amount of hydrocodone and the filler ingredients by the levels in your blood, but in a
urine test, they can not, unless they do a gas chromotography. The
GC breaks it down by the metabolites left in your body and the amounts of each chemical.
If you don't have a valid prescription for either drug, and you are taking both, you will get caught if they use GC testing.