I am now wondering what is wrong with asking the seller to post it directly in Vietnam?
Yes, you should ask. Few sellers don't send items on few countries as there are complaining about the missing of the item. Or any legal problem to send there or something else. And if, so you can ask which country they can deliver. Then you can search escrow for those countries if required.
There might have a lot of sites like this and many of them are a scam. That's why it will not be wise thinking to use any unknown sites. So better to use bitcointalk escrow who are very much trusted. So,