It's not vanishing money. Maybe the npcs use the caps to buy more medicine, supplies and make up his living.
If he was to but medicine for those caps, the caps would reappear in the pocket of the person who sold that medicine. This is not the case.
Big gangs also provide most player interaction.
Right, to other big gangs and random soon-to-be-dead bluesuits trying to mine/get profession/sell stuff. You get caps from TC and those caps appear out of thin air. You could argue that the caps come from taxes etc from NPC citizens, but that's brahmin guano. If a gang locks a town, and kills everyone on sight then there's no income for shopkeepers and suchlike because there's no-one to shop, buy good or services. A town under TC is like North Korea or the Soviet Union in the peak of Stalinism. A system like that dosen't spawn money on the hour every hour, it starves its citizens to death.
My point here is that game should reward player interaction, not solo farming.
So lets scrap rewords from TC, lets scrap NPC vendors, quest rewards etc. If players want to trade let them trade among players and only players. That would reward (the hell out of) player interaction. It is actually plausible and has been done before (eg. SWGalaxies -100% player run economy, if you wanted a x-wing you made one or found someone who could build one for you).