The quality of the item tells you its level. In your case, superior quality.
The ranks of quality are as follows taken from help identify, and I'm pretty sure that the status levels I have attached them to is almost exactly accurate.
Poor (Amateur to Novice)
Average (Initiate to Apprentice)
Above Average (Journeyman to Veteran)
Superior (Expert to Mentor)
Outstanding (Master to Champion)
Near god-like (Grandmaster)
So for your chainmail vest in particular, it's probably in the very upper end of superior and thus the upper ranks of Mentor: You were likely only two advancements at most away from being able to keep it, and even then you're unlucky to have lost it, because the closer your level gets to the item the less likely it is to be stolen.
Keep in mind that enchanting an item also increases its quality, and that
the first ten enchantments for an item do not count as far as level thieves are concerned, so conceivably something that starts out as "superior quality" can be enchanted to have "outstanding quality," yet still be kept by an expert or mentor with no risk of losing it.