User is trying to create purchase orders from the Budgets screen (Projects > Budget > More > View All Budgets). However, the Vendor ID drop-down shows up blank in the Create Purchase Order dialog. When the user is creating purchase orders directly from the Purchase Orders screen, the vendors show up just fine. So, why are they not appearing in the Budgets screen?

The reason vendors don't appear in the drop-down on the Budgets screen is because they have to be assigned to their respective budgets first. After you assign them, they appear in the drop-down.