There is a way to make the Samsung Galaxy Watch work with the China phones from Vivo and Oppo, especially their Fold series that include Fold, Fold+ and Fold2.
For that, you will need to set their Samsung Galaxy Watch that run Wear OS so that they are able to talk with the phones. So, we are going to install WearOS China APK from here and install it. Open it once and close it. Then, to its settings > turn on auto start and set its battery to high usage so that it is not closed by the Android OS when not in use.
Also, make sure to give WearOS China APK app all the required permissions.
Then, go to Play Store and download Wear OS app called WearOS by Google. Go to its settings and give all permissions and ability to auto start. Also, turn on its notifications.
Now, open this WearOS by Google app and set up your Samsung Galaxy Watch. It is important to note that you should never delete the WearOS China app. If you delete it, the watch will stop working and you will have to start the entire process again.