Hi,
You have several ways to go about it:
- You could simply activate the "Unicode Aliases" setting of your Joomla configuration. That way, SEF URLs will be able to have special characters in them and the ộ will be in the URL as the other characters and the links will work.
- You could enter the alias without special characters in each product. If the alias is provided, the system will use the alias instead of the product name when generating the SEF URL.
- The conversion of the product name to an alias without special characters is done with the transliteration system of Joomla which uses your Joomla language transliteration function. So normally, it should be able to convert ộ to o if the transliteration function is properly done. Looking a bit online about it, I found this page which apparently talks about that:
www.vietpublic.net/tin-tuc/tu-dong-chuye...au-trong-joomla.html
However, the text is in vietnamese, which I don't speak, so I'm not sure if that will help but it sounds like it explains how to add the transliteration system to vietnamese in your Joomla.