Quote Originally Posted by Grond
To a man, they all came back with "this man has no vices that we can discover."

I looked at French, Spanish, Byzantine, Papal generals. Clean. All of them.

Then i considered the affect of such success, and concluded that the minor details that might ensue weren't really enough to put the spies at risk. I did, however, notice that a high valor spy seems to be able to ignore watchtowers about 66% of the time.
The only vices that are revealed are the secret ones (secret atheist, secret heretic, secret bloodlover, etc.) so you have to pick your targets. If an enemy general had a vice that suddenly dissappeared, you can be pretty sure he has moved up on the vice ladder, but that particular sport is secret. Then is a good time to investigate him. Generals that survived an inquisition are also worth a look. They do have vices, though certain factions and the human players seem to get them more often.

Repeatedly investigating a general will give him the "informers" vice which lowers happines and loyalty (IIRC), but I think this rather cheesy.

High valour spies are indeed more likely to evade border forts.