I had pretty decent results using enhancement runes (YES, I know, they're a cash item)
However, performing 1 enhancement per day to get my rakum rakish to +11 (10-11 was done without rune obviously) I had three failures So 3 failures out of 10 success is pretty good odds.
Now I'm fully aware that your character has "X" amount of REAL luck upon creation, but I'm also partial to believing that it's a daily thing too. On days you have good drops, try it then, you never know.
I DO know that in world of warcraft, the dungeon loot was based on luck (random as well) but also had variables in place judging from how many persons ran 'x' dungeon within the hour you did. SO if you ran it when no one else was, you had a higher percentage of getting better loot.
Not sure if Vindictus works the same way or not, but I DO know that Contract, and The Last fragment drop WAYYYY more scrolls at night (after about 11pm central US time) than they do in an ENTIRE two days (from 7am to about 8pm)