I was in GC tonight when a n00b got some bad advice about thieves. The n00b asked if a Vault was worth having. Some random player then replied that a Vault isn't very useful against thieves (true), and that building some defensive thieves was an effective protection against theft (categorically false). I took the time to send the n00b a mail describing why that statement was inaccurate.
You received bad advice about defensive thieves.
The way thieves work in this game, if the enemy thief attack significantly exceeds your city's thief defense, the theft succeeds with no casualties to the attacker. Any credible thief player will centralize their thief army to maximize the chance for success, and also to cut the upkeep in half with the Thieves Den buildings. For example, I strike with 65000 thieves from a single city. There is no way for you to maintain enough defensive thieves to stop my attack, and certainly not in every one of your cities. If you build maybe 500-1000 defensive thieves in every city, the best you can do is stop a total n00b who has no idea what they are doing.
The proper way to defend advanced resources is to store them in a nearby friendly trade hub. Hubs cannot be attacked by thieves. You will need a trader to access the goods stored in a hub. The relevant research is under the Trade tab in the research tree. As long as you don't keep any advanced resources in your cities, anyone who robs you will not be interested in making more attempts. You can further discourage thieves by using a general-purpose slaying Rune (the Ward of Destruction tree under magic research) or with a seeking Rune set to thieves (the Ward of Intentions tree under magic research). Anyone who loses thieves and steals nothing of value will eventually look elsewhere.
The Vault is only really useful for protecting basic resources. In most cases basic resources are not worth defending, as they are so easily replaced. There are special advanced cases where basic resource production must be shielded with a Vault, but they are generally scenarios for huge, overbuilt cities or military cities engaged in a ferocious exchange of fire with the enemy.
Hope this helps.
<^^^^^^^^||==O Ten Kulch 
There's not much more I can add to this mail. Defensive thieves are a useless defense against anything but a tiny, useless attack. Any experienced thief is going to deliver an overwhelming strike to a single city. The gold spent on a minimal defense in every city is completely wasted.
Hubs are a free, guaranteed defense against theft. Learn to use them, and never fear thieves again.
Misbehave. Kill lots of stuff.
<^^^^^^^^||==O Skint Jagblade