It's definitely legal for someone on a tourist visa to sign a legal rental agreement which is longer than the duration of their visa.
it's basically up to the landlord as to whether they want to rent to you IF you are willing to comply with their lease terms.
On a one-year house or apartment lease, that commonly means one month rent deposit and 3 months rent in advance.
It's also up to you, in that only you can decide if you trust the landlord and any agent involved to be filing the lease legally with local authorities.
So I definitely do not recommend that anyone simply come to Vietnam and accept an offer to lease a place through someone you don't know, or who isn't highly recommended by someone you trust.
This usually means one of two scenarios:
1. Somehow you have become personally acquainted with a Vietnamese person (Việt Kiều or citizen) who can help guide you through the process and assure you that you are dealing with a reputable landlord and that the paperwork for that property is legitimate.
Often this will be the spouse of an expat you know, or someone you have connected with over the years, developing a high level of trust in the relationship.
A good example was when forum friend Ciambella assisted a number of us in signing leases for apartments in a property owned by her nephew in Vũng Tàu.
2. Through the recommendations of many others you trust: a property agent or lawyer in Vietnam who is arranging lease agreements.
If the landlord trusts you and you trust the landlord and you are willing to fulfill the terms of their offer, you can rent the house or apartment and continue to do Visa runs out of the country and back in again without needing to do anything special other than immediately giving your landlord a copy of the new visa when you return from the Visa run, so they can update property resident records with the local police.
If you are living in a hotel and you want to remain in that room after your Visa run, simply notify the hotel and pay whatever rent they require in advance, and they will usually store most of your personal belongings for you.
Whatever option you are choosing, it's always a good idea to carry your most valuable positions such as laptops and cameras with you as you cross the border.
In the rare case where you are not allowed to re-enter the country, you won't be repeating the few horror stories we've heard of people who got stuck outside Vietnam with some of their most irreplaceable possessions still in their lodgings.
