Even the most organized and planned projects can have ad-hoc purchases that happen at the jobsite. In an ideal world, all vendor expenses would be controlled by issued purchase orders driven by a project budget. Vendor expenses should be recorded as an incurred cost against the original purchase order so that you know all of your budgeted costs upfront and you have full clarity around your accruals.
In reality, a project can have a substantial number of non-purchase order vendor expenses that need to be recorded and accounted for on a project. The right vendor management tools can provide you with full visibility into vendor accruals so that you have accurate project cost tracking.