Amazing help here!

As I replied your message, your questions are too vague to answer. However, I can say all depends on which brands, quality & the quantity you want as well as type of business you are looking for (buying-selling or contract type, etc... please read my reply).

You won't be able to explore the market this time you come to Vietnam. Because if you want to visit factories & talk with the right persons, it's not like you buy a ticket then get in & get out. What you're about to have is a wide view of the market. If you're lucky, you will know about pricing, where should go & whom should talk to, not really specific information. You can only walk around some markets in HCMC where people are selling various types/brands of cloths, shoes, accessories,... Don't believe in your first look, many of them are from China! But sellers will tell you they are from Taiwan, Thailand or left over.

It's best if you can talk to the merchants who are the bridge between sellers & factories if you want to buy branded stuffs in small quantity (more than hundreds). You can talk with the factories if  you are going to be their buyer at big quantity. If you are, choose the brands you want. If you're looking for someone who produce base on your designs, it's another way to head to: tailors, or simply contracted seamstresses.