For this, you will require to create a custom restlet script in NetSuite. The script will contain the above logic mentioned by you.

The restlet script can be coded in the 'GET' part. The design would include fetching the Purchase Order ID (which you will be passing via 3rd party application), fetching the Vendor Bills associated with this Purchase Order ID, and returning the values.

Let me know if you require more details or if you have any concerns.