VPOs in the system can be handled using the CPR (Change Proposal Request) feature in the program. If there are unforseen changes in the job and you can document the potential items in a CPR and will be able to issue a Change Order (CO) for further billing as well issue Sub Change Orders (SCO) or Purchase Orders (PO) to the appropriate Subs / Suppliers in the system.
A CPR will not affect the budget until it has been converted into a CO / SCO / PO.
Hope this answers your questions.