You can see the cover of manga (in Vietnam unofficial version) here (Google Drive).

There are things I know:
+) Has a little similar in plot with Arsène Lupin by Maurice Leblanc.
+) Old, > 20 years, the infringing copyright version in my country (picture) was published in 2002, which means this manga probably was written in 1980~2000.
+) Completed in 4 volumes.
+) The Vietnamese name translated into English is "Knight Robin" but I do not think it relate with the original.

Hope you can help me! Thank you!

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EDIT:

+) This is the only inside page I found: https://drive.google.com/file/d/1cmZEXVsU15VxyxHjJ3IltE94o9TOlp5Q/view?usp=drivesdk

+) It might from China, in 60s~80s (?).

Looks like the only other mention of this series on the internet is this forum thread. I don't speak Vietnamese though so my searching skills may be impaired by that.

https://tve-4u.org/threads/hiep-si-robin-unkown.39014/

Are you sure that this an unauthorized translation of a manga and not in fact an original Vietnamese publication?

Art looks a little similar to "Bagi, the Monster of Mighty Nature," but I don't think it's a manga...just an anime film and the plot isn't super similar to Lupin. The art from this era is hard to get a read on, but that style seems too old for anything passed the 60s-80s (more than likely between 1960s-1970s). Manga in the 90s has a different style of dated.

I agree this might be a original Vietnamese publication, considering that the author seems to be unknown based on the link. The title might have more to do with Robin Hood as well rather than Lupin? Just a guess though.

http://i.imgur.com/LxhWm.jpg (will reformat this some day…)

I agree with flowinmyboat that they might not be manga.

It looks like the 'manga' are printed in a Left to Right format. Since Japanese manga is formatted to be read Right to Left the books you are looking for might not be from japan.

If you have access to the books you can check to see what hand the main character is using to hold his sword. Volume 4 shows the main character holding the sword with his right hand, if the pages inside also have him holding the sword in his right hand then the books are probably not translated from japanese.

(If the pages have been flipped to be read left to right then the characters will all look to be left handed)

Hello, thank you for your reply.

I see the post in the forum you mentioned but it can't help... ಥ‿ಥ

I can ensure with you that it was an unauthorized translation. What a shame but I have to admit that there weren't any comics (in general) in Vietnam that was copyrighted before 2007. The artstyle didn't look like from Vietnam as well.

Another reason why I think it was an illegal translation is because it didn't have any author's name or references to the original publisher.

Though, thank you so much for your helping! Appreciate!

Thank you for your reply!

Yes, I agree, the manga could be from 60s-80s more than 80s-2000.

I don't think it was written by Vietnamese, it was impossible for us in 2002. I searched the manga could related to both Robin Hood and Lupin but it was hopeless.

Although the artstyle wasn't very similar, for a moment I thought maybe it came from China (manhua) and was based on BlackJack/RH/Lupin? Haha!

Though, thank you so much!

Thank you for your reply.

It is a difficult problem to explain. Foreign books when translated into Vietnamese before ~ 2007 were converted from Left to Right to Right to Left, because we read from Right to Left.

But your reply has helped me think more, maybe (though the art was not likely) maybe it came from China? This is the only page inside I have

https://drive.google.com/file/d/1cmZEXVsU15VxyxHjJ3IltE94o9TOlp5Q/view?usp=drivesdk

Well, still not satisfactory, thank you though!