The reason of having those strings in the backend is because at the beginning of the component, Frontend edition was not allowed and to avoid duplication of code and resources, when frontend edition was implemented, the same resources of the backend were used.
Right now we are working in splitting completely frontend from backend and this will include language separation if needed for security reasons (to avoid referring any part of the backend from the frontend).
Anyway, you do not need to translate the whole backend file to get those language strings translated, just do a quick search in the admin file of any untranslated string that is not present in the frontend file and you'll easily find those strings. You can even use the language override tool that comes with Joomla! and this way you do not even need to mess around with text files or translation files.
Anyway, what we really encourage is that our users help us with the translation and if you want to help us with Vietnamese translation, we will be glad in extending your current subscription, please let me know if you are interested, because we will love to have JEvents translated into Vietnamese
Anyway, I can give you a hand if you want creating the language package for easy installation and even add your files to our Project at Transifex so that you can translate in an easier way. This would be awesome too.
Please let me know what you think and contact me with any questions you have via PM if you prefer