Don’t think that model uses firebrick on the back. I believe this part is severely damaged:
https://friendlyfires.ca/products/vermont-castings-lower-fireback-defiant-encore-encore-1308646/?srsltid=AfmBOoqE96g2IRDIkc96MQqIwCKWBtiFY_SwhEiJuWpQfn0XHoEYZ1LW
While the part does appear available (no endorsement of the website above, beware scam sites), if it is indeed the correct one, beware that this type of repair is not for the faint of heart. It usually involves broken bolts that must be drilled out and retapped. If you do attempt it, soak all fasteners with a good penetrating oil for several days ahead of time. You will likely need four hands to get it back together. Reseal all the joints well and replace the gaskets before reassembly. Something probably made that stove severely overheat, bad gaskets are the usual culprit. It is also possible that it was run long and hard in a cold climate, or exposed to water over a long time via leaky chimney. Not as likely, but a very tall chimney and/or one that is frequently subjected to high winds could also be the cause (excessive draft).
Search youtube for a detailed repair video.
I’d probably just look for a new used (quality) stove on Craigslist/Marketplace.
If I was in a bad cash squeeze, I’d probably be willing to to run it (after a chimney cleaning and inspection) under supervision, starting with a small fire and slowly working up, making sure it doesn’t have an air leak that is causing overheating. Those stoves are double walled in most places and I wouldn’t be worried about the stove catastrophically failing in some way. Of course with all necessary smoke/CO detectors in place. But this is me, and I have 20+ years of experience heating with wood. If you have never heated with a woodstove, I’d recommend reading a couple good books on the subject. I’d probably stay away from social media for general woodstove advice, unless it is a specialty forum, as there is a lot of “Fudd Lore” out there on the subject.