Can I use this approval letter for entering Vietnam by land?

No, this type of visa is applicable to air travel only. Three international airports in Vietnam that you can get your stamp with this approval letter are:

• Noi Bai International Airport (Hanoi).
• Tan Son Nhat International Airport (Ho Chi Minh City).
• Danang Airport (Danang).
• Cam Ranh Airport (Nha Trang).
• Cat Bi Airport (Hai Phong).