If you are not yet familiar with the Australian Consumer Law, it is the place to start to understand your rights, especially those regarding acceptable quality and the retailers obligations, and that the retailer cannot fob one off to the manufacturer.
Without being expert in your situation, to use the ACL you have to be formal and use explicit words and inclusions. Anything less is chit-chat as far as your obtaining resolution (since you have already been fobbed off).
Having downloaded a manual for your product and looking through the warnings and cleaning instructions there are a few on p5-6, and p22-23 none that seem to have relevance to your assertion on using only vinegar and dishwashing liquid. A caveat might (not necessarily is) be that if a vinegar solution were not flushed and wiped, that could be an outcome and what they were hanging their defence on. The residue could be ‘activated’ by heat and steam at the next use.
That being written, it would, to me, be improbable and unexpected, given the document I read, that dishwashing liquid could have a detrimental effect, although whether it (and any vinegar) was flushed and dried after cleaning could be important.
If you disagree with their assertion you should use the ‘Letter of Complaint template’ on the site I linked above and deliver it to a manager at the shop where you purchased your oven. You need to include all claims about quality and durability, quote the chapters and verses regarding cleaning from the owner manual and exactly how you cleaned your oven, and what you want and by when per the instructions.
While they offered a discount on a replacement, if you are not satisfied with the quality you have options you may seek from repair to replacement, situation dependent, as well as negotiating for a better discount.
Note the shop cannot send you back to AEG. They ‘own’ the problem and it is up to them to sort it. Harvey Norman franchisees have been fined $100,000’s for misrepresenting their obligations, and letting the shop know you know that could be helpful to your cause.
Please let us know what you decide to do, and how you go, or if you would like anything further for now.