How do I protect the intellectual property of my design?

First off, I’m very sorry to hear that someone has been falsely claiming your work as their own.

                                             Industrial Design in Vietnam 

As you’ve probably gathered at this point, this entire scenario hinges on the contract (or lack thereof) between you and the event organisers.

Given how little information that we have to work with, i’ll answer generally from the perspective of Vietnam copyright law. Regardless of jurisdiction, you are right in thinking that this is flagrant breach of copyright.

This shouldn't be considered legal advice, but that I hope it helps you potentially avoid this situation in the future.

Is it possible to start legal proceedings against the thief?

The answer to this question depends entirely on information that you haven't included in your question. You said that you were the official designer for the event that the poster was designed for, so were you employed by the event organisers? or was the work commissioned? Is there a clause in the contract that states that you retain your ownership of the copyright despite it being work for hire? Each would point to a different set of options.

Basically it comes down to three potential situations:

If you are employed by the event organisers in all likelihood the work will be deemed to have been carried out in the course of your employment and thus under a contract of service and copyright will lay with your employer.

If are you are an independent contractor it is possible that the work was done under a contract for services in which case, it is possible that you retain the copyright ownership (unless the contract stipulates otherwise).

If you were commissioned to do the work you likely retain authorship unless the contract stipulates otherwise.

If you have retained authorship:

You are well within your rights to proceed with legal action against this person for breaching your copyright in the work.

If you don’t own the work:

You will need to try and get the event organisers (or whoever does hold copyright in the work) to bring an action against this guy.

OR you can attempt to sue him for breach of your Moral Rights-

As the creator of an artistic work, regardless of ownership, you have the right to be recognised as the creator (right of attribution).

To go down this road you’ll need to be able to prove that you are the creator of the work (easy enough with the meta-data from the files, the exchanges you will have had with the event organiser who asked for the work and the payments made for the work etc).

How can you protect your work in the future?

This might sound obvious now but include a watermark, send low resolution files for approval (BTW - how did this person get the file in the first place? Maybe you need to have a chat with the event organisers about ‘circulating’ files of this kind).

It might be worth considering including clauses in future contracts that retain ownership of the work thus putting yourself in the strongest possible position to challenge any infringers.

How should you confront the guy?

Since you know who this person is, start with a direct communication between the two of you, tell him that he is breaching your copyright and that he needs to stop. Dont go full on litigious unless you have no other options because it’s guaranteed to be drawn out and expensive.

However, if he doesn't care/respond then ramp it up: have a lawyer send a letter on big, scary law firm stationary demanding that he stop or you’ll start the appropriate proceedings.

And if that doesn't work and you’re in a position to do so, take him to court for breach of copyright.