visa for a six month stay in vietnam

I am a spanish citezen, am going to Vietnam to marry my Vietnamese fiancee. I wish to stay in Vietnam for six months, to get to know her family, the vietnamese, and Vietnam, a country I admire very much. I understand that the visas are extended for three months only. Is it not possible to obtain permission to stay for six months? Thank you for your attention, awaiting your kind reply, Francisco-Rui da Cruz Costa