I’ll fully admit to not being a flex expert (really should learn someday)… but wouldn’t vertical-align be assigned to the element containing these two elements, rather than the elements themselves?
align-items:center will center flex-items vertically but of course you can’t centre text exactly because space is reserved for ascenders and descenders that may be present, The font may also be created with more space at top than bottom (leading half/leading etc).
Vertical-align only applies to inline elements, inline-block elements and table-cells. It does not apply to block elements. Usually you apply it to each inline fragment so that you can align it with the next in line :).