What I did while mining in regards to a Thorax was I bought one Thorax and then bought a combat fitting and a mining fitting. If you fit enough Miner II's you can get close to the amount of ore that you would get with a Mining Barge.
Then you can take off all the mining lasers and such and fit your railguns or whatever you bought for your combat fitting.

I havn't tried using a Vexor to mine with because I'm now using Mining Barges but that's just my personal experience with Thoraxes