What kind of companies need these services in China/India/Vietnam? What's the best way to reach them? Ex: google ads or cold email and how do I find out if they already use such services?
Well, every company needs these services, but only some of them need it as an outsourced service. Most attempt to handle internally. To reach them you need to be in a position where you are speaking with authority about the importance of these functions, and how internal handling is too risky not to rely on outside expertise.
The problem I see is not that you don't know who needs the service or how to capture their attention with advertising. Rather the issue is these companies who need these services don't believe they need these services. You need to change the belief system.
The reason buyers complain about product quality from these sources is because the companies don't believe that quality is essential. It's usually all price-driven. Your real customer should be the buyer, not the manufacturer, companies that want low prices but also a certain quality assurance. The challenge for buyers is being able to communicate the expected quality standard.
There's a whole rigamarole of ordering samples and not finding satisfaction so the buyer keeps trying to find other sources. It's time consuming and frustrating, and the buyer simply rejects the manufacturer rather than coaching them into meeting an acceptable standard. Highly inefficient sourcing.
I'm going through this myself right now. Fortunately I know how to speak the technical language that leads to a very clear specification for what I want to buy. But I'm still encountering language barrier issues and trust issues. The manufacturers are only interested in the quantity and think all I care about is the price. Really what I care about is the value. I have a minimum quality standard and I'm looking for a fair price to meet that standard. It's not easy.
I wish you lots of luck, but I think your target should be buyers, not manufacturers. If you are the communicator and can carry deals in your pocket, manufacturers will start focusing on the things you tell them are important to win those deals in your pocket.
Stay in touch if you figure it out.