Tam Tru

  yes. You will need to do Tam Trú when you live in Việt Nam! Includes tourists visa. It mean you let police know where you leave when you here. It very easy.

"Tạm trú" means temporary residence.  Each time I renew my family visit visa, the village police must sign the application to state that I am registered as a temporary resident in the village.

how much does it cost and where can i get that one?

Ask for a 'visa thăm gia đình' at your nearest Immigration Police office ( Sở cảnh sát xuất nhập cảng)  or visa service office. This is a multi-entry visa for six months or more. It costs around $130USD [depending on how much the local Immigration office wants to overcharge you].

It is odd that a private visa service can often get you the visa you want, whereas the Immigration Police will refuse to issue it.  Welcome to Vietnam.

The owner of the hotel or the residence that you are staying at should be doing it for you.  You cannot just walk into the local police by yourself to do it.

yup MegaMettle is right, this thing is initiated by ur hotel or residence owner, ask them. Some friends who were staying in a hotel, got this made for them for free and in our case, we were told to give 500.000 vnd to our landlady to get it done. That's how it is here in hanoi.