Opiates stimulate histamine release which causes your bloodvessels to dilates causing a rush of blood to your head, making your face and eyes look flushed, along with the itching.. This response it dosage dependant(in other words, the more you take, the more vasodilation).. So as you tolerance grows(over 5 years) you have to take more, and a larger amount of vasodilatoin occurs.. It may just now get at the point of noticing..
Still, 5 years is a very long time to be habitually using anything with tylenol in it... Id suggest taking a long break.. Trust me, you dont want liver damage.. You also probably want to get a few simple tests to asses if there is any damage, and maybe take Milk Thistle as a supplement for a few months(Milk Thistle has been shown to help repair and prevent liver daage).. You can find it at Walgreens..
Also, if you feel you cant quit right now, at least move to oxycontin, methadone, or even snorting(no shooting) heroin to prevent any further damage(shooting can aggrevate liver problems)..
Anyway, Id assume the flushing has nothing to do with liver problems.. However, liver problems can be symptomless for awhile, and your past use of vicodin may warrant a checkup.... Better to catch a problem early and take pills for it, than have Hepatitis or cirrosis later. If you dont get a checkup, at least watch for dark yellow, bloody, or amber colored piss. This and yellow tinted eyes can be a sign of liver damage..
Good luck!~