Player vendors work very much the same as they did on OSI, with a few modifications.
You can place a vendor in a house you own with a Contract of Employment. You can also rent a vendor in your house with a Vendor Rental Contract. Both types can be purchased from an innkeeper or a minter.
When you double click the deed to place a vendor, it will stand where your character is standing and will not move from that spot. It will start with 1000 gold in its account. There is a daily fee, which is charged every UO day (a UO day is 2 hours). When the fee is charged, it will be deducted from the account.
When you double click the rental contract, you will be prompted to target the floor where the vendor should go. Afterwards, click the deed and select "Contract Options" in the context menu with the house owner. On this menu you can decide how much to charge for the rental, how often, and who to rent the vendor to. Once the vendor is created, you can click the vendor and set it to renew at each interval automatically in the options.
Be sure to check out the Shopkeeper Profession page to learn how you can enhance your experience on Angel Island as a vendor enthusiast.