I've been playing since release and I've never gotten a single one. I've only clocked ~1500 hours though... comparing that to some people who have been playing this game as long as I, it's not much time at all. Still, you'd think I would have gotten at least one...
~2400 hours (I think, over 2000 I know)
I've gotten... I believe 3 or 4 (might be confusing one of those with a superior absorbtion back when they were worth 100k).
I've been playing since November 2006 (9 months (maybe 300 hours?)), have identified about 60-70 gold items (mostly random drops; maybe 20 from chests), and gotten one sup vigor.
Luck of the draw, really.
9 months playing, got 3 or 4 Superior Vigors in the last 2 months, never before.
One of them I got from a guy selling a unide armor on Droks, bought it for 500g, trew in the storage and forgot about it. ID'ed it yesterday and got a Sup Vigor and Radiant Insignia hehe.
I have had about 12 - 15. Mostly from chest running. Twice I have had 2 in a single day and once it was like 4 in a 5 day period. They are more common than you think.
Most of them have come from Ice Floe and Mirror of Lyss.