Standard SAP
You can create value contracts w.r.t vendor & then convert the same to PO with limits.
Development
Or if you want a limit to be kept for vendor to purchase i.e no open orders more than 1lakh be allowed against any vendor.Then this requires development.
You can start with creating a ztable storing value limit & vendor code & then you need to calculate the total open value of PO from EKKO/EKPO & write logic in user exit to trigger an error if a PO is ordered above the specified limit.