Send email to Vendor with Purchase Order pdf attachment

Question is: How to setup Vendor record and understand best options in Purchase Order transaction to efficiently send out Email to Vendor with Purchase Order pdf attachment.

Use Case is

Using both Standard and Outsourced Purchase Order transactions

Custom forms for each PO type are created with custom Print and Email PDF/HTML Templates assigned in custom Purchase Order forms

Custom Email Message Template is also created and assigned in custom Purchase Order forms

Vendor records > Preferences > Email Preferences are set to PDF

Approval routing is NOT used

Testing completed for the following processes

Tested marking SEND TRANSACTIONS VIA EMAIL box in Vendor A record and when created/saved PO for Vendor A an email was sent, but did not use the Custom Email Message Template assigned in Custom PO form and did not have pdf attachment. Instead email was sent with copy of print PO PDF in body of email? Is this expected behavior when marking VIA EMAIL in Vendor record?