Do this (ideally with a fresh data file if possible):
Create a new vending machine profile.
Create a new listing > Select the STIP item that you have added to the default currencies field in the config.
Add it for sale for 1 scrap
Create a new listing > Select scrap.
Add it for sale for 1 of the STIP item that you added to the default currencies field in the config.
Spawn the same STIP item using the stip.give command and add it to the inventory of the vending machine.
Create a new listing > Select the STIP item itself (not the currency) to list for sale.
Add it for sale for 2 scrap.
Drop the data file and config files for CustomItemVending in here.
I did some testing, and it seems to work on my server. There was an issue when I initially added the default item to the config, where I had accidentally had xp as lower case rather than upper.