PA credits where tax rates could have changed from original transaction date

System setups:

We invoice using PA and credit via the invoice review in PA.  We have the profile AR: Use Invoice Accounting For Credit Memos set to YES, and the the setup for the specific transaction type in AR has the Tax Calculation falg set to NO.  We are also using Vertex O series as the third party tax engine.

Issue:

Our credits seem to come back to us several months, and in some cases, quarters after the original transaction occured.  Is there a systematic way to send the original transaction date for the tax call or do I need to customize our Vertex call for credits to ensure I credit the same tax rate?