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Go to the markets, lots of cheap authentic Vietnamese and Chinese and maybe Thai clothing.

By authentic, I'm guessing you mean big name brand labels like you buy in US malls, but you didn't say that. No, those contract factories manufacture for export only. The brands would be really pissed off, it would undercut local Vingroup malls, it avoids taxes, etc. 
For example, I have never seen Patagonia in a market or a mall. The malls will have real Nike and North Face etc for a lot more than Amazon. The markets will have fake Nike and North Face from China, and the quality is pretty good for quarter the price.

If you like good clothes cheap, try Bangkok, amazing. I hear that some Vietnamese import from Thailand.

Hi Young man,

Here are some sites to get discount & they will deliver to you if you order the product. You can pay online or cash when deliver man sends the item

1.
2.
3.
4.
5. Yes24.vn
6. Lotte.vn
7.  Adayroi.com

and this site is to compare the product to see where is selling cheap one. But it seems they are unable to cover the market fast so don't take much of trust on this site

Here in the south most shoe factories are located in Binh Dung and Dong Nai.
In the north I heard in Hai Phong there are some factories.

As for garments, you can visit Vinatex on Ba Trieu street (25 Bà Triệu, Hano). They have wide range of garments (both cheap/discounted and expensive), all made in Vietnam.

You can search to find shoe ( and garment) manufacturers in Vietnam.

Welcome to Vietnam.
Cheers!

The only stupid question is the one that's never asked.

Fools are full of Answers: The Wise full of Questions   - Socrates

Thanks everyone for all the great replies. Specifically looking for Hoka One Ones. They’re excellent running shoes, but it sounds like I’m going to have to pony up and pay full price (they’re expensive). I’ll go to the mall at Times City in Hanoi and see what I can find.

Appreciate the help!

Jonathan