Compare Vendor Bills Date to Customer Invoice Dates

I'm trying to come up with a search that will help me tie out the end of my month. Specifically I'm looking for Vendor Bills which are dated towards the end of the month, but where we didn't bill the customer for the goods until the next month. Dealing mostly with dropship items on this., so we may place an drop ship PO on April 28, vendor ships and bills us as of April 30, we get the bill or shipping notification on May 2 and that's the date we bill the Sales Order.

Trying to identify the transactions in this scenario, but so far I've had no luck. I'm starting with the Purchase Order, as its related to both the Vendor Bill and the original sales order. From the Purchase Order I can reach back to my Created From SO and get the current status, but the date billed field is not used, so there's no data there, and it seems I cannot get to the system notes of the SO where I could look for a document status change. Having similar problems reaching from the PO into the Vendor Bill, where I