"Vomitorium is where you dispatch Caravans. Upgrading the vomitorium can increase the number of Caravans and reset today's commerce."
The Vomitorium is available at senatus level 41. It allows you to send caravans to other cities, earning both players a bit of silver.
Open the commerce menu, which displays various information about your caravans, and allows you to manage any trade applications. Just like the market, remember to ship out your caravans before upgrading; the amount of caravans you are allowed resets each time you upgrade.