Without argue the hands-down best thorium farming area is EPL, specifically the north, north-east and east areas of EPL. In an hour's time I will farm app. 10 to 12 stacks of ore, along with the normal amounts of gems and stone. But to be sucessful at farming mines you need to keep a few things in mind. The game software works random mine appearances in not-so random ways. As everyone knows there are specific node locations that mines appear at. There is more to it than that though. First the entire Zone will only have a certain number of active mines at one time. I.e. x-nuimber of RTV's, x-number of STV's, x-number of MV's, and each of those has a small percentage of being a TSV. Respawns of these also cycle in what appears to be ten minute increments. So let's say you run around EPL and mine 7 RTV's in 5 minutes. If you continue searching you will be wasting the next 5 minutes searching in vane. Another part of the game mechanics is that spawn nodes are grouped in specific areas. For instance in EPL there are RTV node groups specific for Noxious Glade, Infectus Scar, Zul Mashar, along with others. Now, each node group consists of appearantly half a dozen spawn nodes, out of which only one vane will be active at any one time from each group. Being that there are 7 RTV node groups in EPL, then only a max of 7 actual vanes will be active at any one time. Of course this is all pending any other players mining at the time.
A couple other notes from expeience;
1 - First and foremost, keep in mind that WoW is a game. This denotes competition and winners & losers. That is what a game is all about. So when you get "ninja'ed" on a thorium vane, suck it up. You lost that one competition. You can win the next. IMHO there is no such thing as ninjaing when it comes to mining, herbing, ect...just a matter of going for a win.
2 - If you happen to be CREATING a character based around farming ore and/or herbs, there is no better class than a hunter. Regardless of the responses I get here to the contrary, keep in mind that those replies are based on people's favorite classes IN GENERAL. For mining and herbing, nothing beat a hunter with feigning death, using a pet to control agro, and freezing trap.
3 - Mount speed is a definite advantage if you plan on farming at length.
4 - Being friendly with Argent Dawn is a plus when farming in EPL. The AD dwarf at LHC is the only vendor in EPL that repairs. And being that he stands right next to a forge, it a convenient spot to repair, smelt, and sell trash loot all at the same time.
Hope this helps...:)