Since the update all my custom vendors are selling the same default clothes items instead of custom items. My data file (Default.json) still has all the custom items but it's just not working.

Custom Vending Setup is responsible for applying the customizations, whereas Monument Addons just saves the data and passes it back to Custom Vending Setup. I'm not seeing any other reports like this on either plugin so it doesn't seem to be a widespread issue or an otherwise systematic regression since a recent Rust update.

The first thing I would try is reloading one or both plugins. If that doesn't work, I would try ent killing the vending machines and then reloading Monument Addons to see if that mitigates the issue. I would also check for errors in the server logs, as something may have failed when trying to apply the customizations, and the error message may give me a pointer as to what could have gone wrong.