This might be very obvious, but I myself did not think of it until after a lot of gameplay (including Fallout 3 and New Vegas).

With vendors or NPC's that repair armour for a price, one can, as far as I know, pick-pocket the money transaction after such transaction (of paying to repair the armour).

With this principal, a player could repair their armour for free, in a sense, and also do the following: bring loot (weapons and armour, only) for repairs, retrieve the spent money, and then sell the loot as one usually would, except in near or maximum condition (much more money obtained).

They've actually made it VERY difficult to successfully pickpocket a large sum of money from vendors.

This. Even with 100 sneak it is incredibly difficult to steal sums of money in the hundreds. Sitting there and constantly reloading would appear to be your only option, I assume there is no cap on the difficulty of pickpocketing so you'd have to do it in small amounts as well.

Basically, the hassle isn't worth the reward, money isn't as free as it was in FO3 but you can still amass goodly amounts if you're good with loot.

If you want to make money, get Jury Rigging, buy expensive items in bad condition and repair them with cheap items. Buying a near-broken Super Sledge and repairing it with a couple of Pool Cues nets you several thousand caps for the investment of 30 caps or so.