I filled out the form, found at the Enter Croatia site, and submitted it. Back came an Announcement of Entry form, which did assign me a number. I submitted proof of my double vaccination, all indicating timeliness, completeness (by date and number), and on the CDC card.
My question is whether this is all I need. It seems so, but I saw an earlier blog exchange on this about needing something with a bar code. This form does not have a bar code, just a number.