Went to Greece in June with layover in Amsterdam on the way and Munich on the return on their respective airlines. Needed a PCR test within 72 hours of arrival for Amsterdam, vaccine card didn't mean sh!t, but Greece did allow the vaccine card. On the return, rapid antigen was okay for returning to the US. IIRC Lufthansa only cared that you fulfilled the requirements to enter the US because the layover in Germany was <24 hours.I don't know where you went but EU and the airlines only accept PCR tests.
And some countries don't recognize US vaxx cards.