(I'm in Đà Nẵng, but I think my experience probably holds true across all of Việt Nam)
In one day, my rent dropped ₫2,000,000, my hotel room size tripled (with an extra bed!), my seaview became totally unobstructed and the price of my meals decreased ₫50,000 on average.
How?
I moved FROM the Expat/Foreign Tourist area of Đà Nẵng (Mỹ Khê Beach/An Thượng) to the "local" (Vietnamese) tourist area of Đà Nẵng Bay/North Đà Nẵng.
Everything here is less expensive. The biggest price you pay for living here is the lack of signage in the English language, and fewer merchants who speak English.
However, help is available if you are willing to look for it.
I have a Vietnamese lady friend who lives nearby. She has been helping me learn to navigate the local market and restaurant menus.
Also, the family owners of my hotel all speak some English, especially the mother, who works as a manager at a foreigner-friendly resort across town.
My expenses would be even less if I didn't insist on being close to the beach WITH a sea view:
I'm paying ₫5,000,000 (about $215) per month for this 4th floor bay view and beach access across the street. Normal daily rate during the off-season (now) is ₫300,000.
However, a room in the rear of the same hotel is only ₫180,000 daily, so you could surely negotiate a monthly rate around $150.00, or less.
FYI, these are not kitchenette apartments. If you need that, you have to probably rent a block or more away from the bayfront.
*I think*, if you are willing to explore areas away from expat/foreigner comfort zones, you will be rewarded with lower cost-of-living expenses AND a much more authentic Vietnamese cultural experience.