Hey and welcome @emmker
This is not necessarily a 'bug'.. Previously the plus button would open a modal window, which would then refresh the page and you'd lose the info you'd entered (unless you're in Firefox, which is cool and saves fields).
Currently the vendors are echo'ed out onto the page as dropdown options. This is a point in time operation, so any vendors added after the page load wouldn't show up.
In an ideal world we'd pull the vendors via ajax.php, possibly with a button to refresh, so the new vendors show up straight away. We'll leave this open until we can come to a solution.