Apartments

Rent until you have an idea about real estate here.

You can rent a serviced apartment for around 10 mill VND (roughly $500) a month.
Check this site (for reference only) https://batdongsan.com.vn/.
The best way to find an affordable and decent accommodation is to go for a local search after you arrive in Vietnam.

I wouldn't ask for advice from real estate agents on an expat site if I were you.  Now that you've expressed a desire to rent an apartment, you'll be hit with more friend requests from RE agents than you can imagine.  Do what you want with those requests, but ask yourself, would those complete strangers still be interested in you if you don't have the one thing they're after?

Senwl has given you a great site.  I would just stay with that one site and forget about the rest. 

Your idea of $700 is not "on the lower side".  The low end is $350, or 8M.  Of course, it depends on what, where, who, and how. 

What: You pay less if you live in a building with few amenities. 
Where: You pay less if you don't live among expats. 
Who: You pay less if you rent directly from owner.
How: You pay less if the building is not brand new.   

One thing to remember, not all one-bedrooms are created equal. 

Most so-called one-bedrooms are actually studio apartments.  I'm from the States so I don't call a bedroom a bedroom if it doesn't have a door to completely separate it from the living area.  A real bedroom costs more.  On-site gym you can use for free, balcony you can actually relax in, a safe neighbourhood, a street that is both convenient to everything you need but removed from all noise, etc., all of them may or may not add to the rent.

BTW, you can bargain on the rent if you rent directly from owner so that's something to keep in mind.  In addition, it doesn't matter if the rent is quoted in US$, you'll pay in VND.  That means you'll not pay the exact amount each month in VND, as long as the amount equals to the same US$ stated in the contract.

You cannot know those things until you arrive here and see them with your own eyes, so forget about buying or renting or even checking things out from CA.  It's really a waste of time.

Ciambella's is the correct answer.

I will only add some frosting by noting:
- most apartments come furnished
- one year leases are standard but some places will take shorter at higher rent
- use a Chrome browser when searching Vietnamese ads, the translator will activate. You can use a map and start to get familiar with the districts.

Rent before you even consider buying.

It'll cost you less and you will get a feel for the behaviourial traits of real estate agents.

Should provide realistic exposure to the ethics involved in the real estate industry before any further financial commitment is made.