Okay, that’s an interesting topic. Let me give you some advice.
First of all, do not set anything up before you arrive. I also suggest your investing up to 15$ a day to stay in a hostel while searching.
Secondly, if you’re not fluent in Vietnamese, you better hire an interpreter who will accompany you throughout the process of searching and negotiations with the real estate agents, landlords and so on.
Also take into consideration that often landlords require a deposit for a long-term rent, but my advice – never agree to pay for more than 1 month ahead. You better prepare about 1200$ just for that purpose. It’s not a must, but letting you know just in case.
As for the price, I seriously doubt that it’s possible to find a sea view flat for 200-300$. It would cost at least 500$, okay 400$ but in this case the beach will probably be in the walking distance. For 200$ you might find a flat with commodities excluded. It means that you may have to set up a kitchen, buy some domestic stuff and pay for utilities on your own.
I hope this helps! Good luck