We sadly will need more than the IMEI numbers to be able to locate the correct firmware from the World Wide Web. The thing is, AG Mobile import some of their phones (ready made and with alphanumeric model #s assigned). They then make few modifications or personalize the software thereof and assign alphabetic model #s. With that said, it is easier to locate the firmware of their devices using the original alphanumeric model #s (which in you case seem to have been concealed.
The same alphanumeric model may also be extracted by navigating to Settings > About phone > Model number... (only unfortunate that you do not have access to the Chrome Vibe any longer). We sadly do not have a Chrome Vibe on hand to made a back from but home to do so as soon as we do. Because waiting for a similar device may take a bit longer, you might want to consider taking your device to AG Mobile who sadly do not want to post their devices' firmware publicly.