Take the number of the Vietnamese person, save it into the mobile and then save it on WhatsApp and then call. Or, if you have truecaller, then see the options after saving the number and you will get a WhatsApp call.
You will need an internet connection, but not necessarily mobile data. Make sure you're connected to an internet source before you update so it won't use any mobile data.
Yes, you can pair two different WhatsApp numbers with the same laptop. You can open one of them in the current window while the other either needs to be open...
The official WhatsApp Web version doesn't support adding a new contact. But you can use some tricks with a third-party extension InTouchApp to add new contacts.
