At the bottom of the definitions in the online Vietnamese-English dictionary is the link to "Thêm từ/Sửa chữa - Add/Edit:".  What a great tool!
 
It has a glitch, though.  When I push the button, it opens a web form, but the word for which I am adding a definition is mangled - the letters with dấu are converted to "??".  This means I have to correct the spelling of the word I am defining before adding a new definition.
 
A little off topic: I have been using the "Thêm từ/Sửa chữa - Add/Edit:" to indicate typos in the original definitions (of which there are a lot in English).  Is this appropriate, and will the typo corrections ever be properly integrated?
 
Thân mến,
John