[Vuejs]-Why does TinyMCE deletes my custom mpdf html tags?

I suspect you are not using extended_valid_elements correctly. You don’t repeat the tag multiple times in the configuration. The documentation has working examples:

https://www.tiny.cloud/docs/configure/content-filtering/#extended_valid_elements

For example:

extended_valid_elements : 'img[class|src|border=0|alt|title]'

…or…

extended_valid_elements :'setpagefooter[name|value],pagefooter[name|content-center|content-right|footer-style]',

Note that you put all the attributes in one set of brackets. Here is an example that appears to work for your example content:

http://fiddle.tinymce.com/iehaab