VietnamVisaservice.com is a Non-Government/ Commercial website operated by VietHoliday Co., LTD. You can either apply for a visa though a Vietnamese Embassy in your country. The price of getting a visa at Vietnam Embassy depending on the country wich you are living.  You are advised to contact the embassy of Vietnam for procedure.

You can stay in Vietnam during time permit in your visa. Then you need to extend it 1month tourist visa or 3, 6 months, 1year business visa

Longer time you can staying in Vietnam if you do a long term visa 1-3 years visa as known as permanent residence.