Weidian is known as "WeChat Micro Store" in China, and every store is linked to a WeChat account, so it should be quite simple to find the account, right?
Actually, there are mistakes to be made, because some sellers setup their Weidian with a throwaway phone number, so here's how I would do it.
1. Seller Profile Page
Click on the WeChat name.
You click on it to open the detailed contact page, because this might not always be right.
2. Contact Details Page
As you can see in the second circle in this example, the seller mentions another WeChat name in the description.
Sellers might do that to keep their chatting separated from their store, which might get taken down at any time.
(In this specific case it actually leads to the same account, one is the ID and one the username.)
3. Solve Captcha
Clicking "click to view" at the top of the page gives you a captcha.
This captcha should be easy to solve. Afterwards you should see the WeChat the account is linked to. This should be the same one you saw when you were at the first step, so you might as well just go back, but this is just as quick.
So now you double checked and you can be sure you got the right account.
4. QR Code
If you go to an item page (this is important) you can scroll all the way down to see a QR code. Scanning this also adds the linked account.
added: 6/26/2023