PLF is no longer required as of 4/22/22. England does not require testing from vaccinated travelers.
Per pt.usembassy.gov, it is unclear if testing is required, so they advise it:
“No specific guidance exists indicating if US CDC vaccine card are accepted for proof of vaccination. As such U.S. Embassy Lisbon strongly recommends all U.S. citizens travelling from the United States to Portugal follow option 2, present a negative COVID 19 test before boarding (see below)”
Per visitPortugal.com, the USA card does not meet the reciprocal vax card arrangement, so a negative test is required.
Per the TAP airline site (which uses Sherpa), vaccinated US citizens require a negative test.
In short, it appears that it’s simply what your airline is accepting as proof. US airlines may be okaying the US card, while TAP (a Portuguese airline) is definitely not.