Yeah, it is
You can mix any dose really... just remember the acetaminophen (aka paracetamol or APAP) in the vicodins become liver toxic at over 4,000mg a day or so (everyone is different though)
and the recommended maximum APAP dose for a single dose is 1,000mg
I would give you the long respiratory depression speech on when the coke wears off... BUT... I doubt you'll die from a hydrocodone overdose in vicodin... you'd probably die from APAP hepatoxicity before that
EDIT: To sum up what I said, yeah, they're safe to take together... just watch your intake of the APAP in the Vicodins