Why do procurement teams reject VPAT reports?

Best practice: Treat Section 508 as its own legal standard, even when WCAG is used for testing.

When accessibility issues are detected, effective 508 Remediation ensures your digital assets are updated and optimized to meet accessibility standards.

A VPAT (Voluntary Product Accessibility Template) now becomes a standard document used in the US to test your WCAG 2.2, Section 508 compliance. 

Which testing method you should use for WCAG certification?

WCAG certification and the VPAT ACR are different but related.