I currenlty have custom vending machines at the train stations. However, while I like that they're in the same place on each one, they all have the same contents. When I use the custom vending plugin in conjunction with this one, it updates the contents for all of the train stations. Is there a way to get different contents for each of the addon spawned npcvendingmachine's?

There is not currently a way to differentiate train stations in Custom Vending Setup. It is possible to improve upon the plugin to allow different settings per monument instance (i.e., per individual train station). However, that will need further design. The main problem is that the plugin needs to be able to tell which orders you want at each train station. That becomes a challenge when you consider that the plugin is designed to persist customizations across procedural maps for multiple map seeds.

Here are some ideas of how this could work.

1. First possibility is to let you disable synchronizing vending machine settings across duplicate monuments. This could be configured per monument type, so you could disable it for just train stations if you wanted to. One disadvantage with this approach is that, if you are using procedural maps, you will have to redo the customizations each wipe. One of the design tenets of my plugins is to save you time so that you don't have to do manual setup each wipe, so I'm hesitant to provide this solution. The only way I see this approach making sense is if you are using a custom map.
2. Second possibility is to let you customize vending machine settings based on the exact prefab of the train station. There are actually 8 variations of train stations: "station-sn-0", "station-sn-1", "station-sn-2", "station-sn-3", "station-we-0", "station-we-1", "station-we-2", "station-we-3".  The only difference between these variations is the rotation. Realistically, I would probably simplify this down to 4 (south->north, north->south, east->west, west->east). One advantage to this approach is that it works across map seeds. One disadvantage is that you don't have complete control and it may not be very predictable since the train tunnel layout is procedurally generated.
3. Third possibility is to associate the vending machine settings with the monument that the train station is attached to. For example, if you edit a train station vending machine that is connected to Power Plant, those orders would only apply to other train stations that are attached to Power Plant. This is possible since every train station is connected to a monument. Even the bunker entrances are monuments. One advantage to this over the 2nd approach is that it becomes more predictable to the server owner and players. In terms of disadvantages, this adds some complexity, and we need to think through how to make it clear to the server owners how this is working.
4. Fourth possibility is to allow you to configure multiple different sets of sale orders, and the plugin will randomly apply them throughout the train stations in the map.

If I were to have any say in this, I'd prefer #3. I like the idea of picking items for that train station based on the difficulty of getting into the station top side.