longer stay in Vietnam

Hello, i have been many times in Vietnam for business and private reasons. 2 years ago I met a girl and since last year we are engaged. I am not sure when will we marry, but for the moment i am thinking to buy a small house. the house will be on my fiance's name. i am old enough, i know what i am doing and taking a risk by my free will. however, a question is: is there any way to stay, say, 6 months or longer for private reasons (tourist visa?) or there is only 3 months visa? If I have 3 months visa does it mean i can stay in Vietnam for 3 months? also (this is not your field, but just asking), in your experience, is there any problem we stay in the same apartment since we are not married? i heard some stories that police made problems for that and many other where there were no problems at all.