It must not be hardcoded. We are able to check the prerequisites of each upgrade and compare them with the IDs of the “vendor upgrades”. For example the “Guild Armorer 1” (ID 38) has 350 as prerequisite. 350 is the “Market Restoration 1”, so we know the Guild Armorer is part of the Market. Sometimes we must check the prerequisite of the prerequisite to get the “vendor upgrade”, but this technique totally worked for me. The only exception is the “Further Exploration” (ID 641) which has the Tavern as prerequisite but a special vendor. The only hardcoded part are the IDs of the “vendor upgrades” but all other upgrades are modifiable.