The following cities have direct flights to Noi Bai and Tan Son  Nhat airport:
Bangkok, Hongkong, Singapore, Kuala Lumpur,  Manila, Melbourne, Moscow, Osaka, Paris,  Pusan, Seoul, Sydney, Taipei, Tokyo,  Beijing, Frankfurt, Fukuoka, Guangzhou, Kaohsiung, Kunming, Vientiane, Luang Prabang, Siem reap, Phnompenh
If entering through Da nang airport, you can only fly from Bangkok

In case of no direct flight from your city to Vietnam, the best way is taking a flight to Bangkok, Hongkong, Singapore or Seoul and get a connecting flight on arrival. As they are four  greatest hubs of Asia (many flights daily), you can depart at flexible time and have significant savings on ticket prices.

Below are some major airliners flying to and from Vietnam which you can search for information and reservation:

2/ ENTER VIETNAM BY SEA

Traveling by scheduled Cruises , you  can enter Vietnam via some main Vietnamese seaports such as Saigon, Danang, Hue,  Nha Trang, Vung Tau and Hai Phong.
Belows are some cruises and website for your reference:

Star Cruises, Diamond Princess,Royal Viking Lines, Crystal Cruises, Leo Star Cruises

princess.com
starcruises.com
crystalcruises.com

3/ ENTER VIETNAM BY LAND

Vietnam has common border  with  China in the North, border with Cambodia and Laos in the West.

You can enter Vietnam from Cambodia via following border:
• Bavet (or Moc Bai- Tay Ninh)
Moc Bai Border Gate is located about 70km northwest of Ho Chi Minh City.
• Kaam Samnor (or Vinh Xuong- Chau doc)
Vinh Xuong Waterway Border Gate is 30km north of Chau Doc Town, An Giang Province. You can take the speed boat from Chau Doc to Phnompenh or vice versa daily. It departs Chau Doc around 7:30 AM and arrive in Phnompenh pier around 12:00PM. The speed boat leaves Phnompenh around 12:30 PM and arrive in Chau doc around 4:30 PM. For the route from Chau doc to Phnompenh, the visa in Cambodia is available at the border. The immigration procedure is very simple with original passport, 2 passport-sized photos and 20 USD/person. For the route from Phnompenh to Chau doc, visitors are required to get Vietnam visa in advance in Cambodia or have visa in hand before the trip.
• Xaxia (HaTien)
Loacted 70km from Ha Tien Town, Xa Xia Border Gate connects Kampot, Cambodia.
• Phnom Den (or Tinh Bien).
 Tinh Bien Border Gate is 25km southwest of Chau Doc Town, An Giang Province. From Long Xuyen City in An Giang Province, along the highway 91, across Tinh Bien Border Gate to Cambodia.

• Donsavanh (or Lao Bao);
From Hue transfer to Dong Ha (Quang Tri Province), along highway 9, across Lao Bao Border Gate, direct to Savannakhet (Laos).
• Keo Nua  ( Cau Treo ) Border Gate in Ha Tinh Province:
Keo Nua (Cau Treo) Border Gate is at the foot of Keo Nua Pass. Buses from Hanoi to Hong Linh Town (Ha Tinh Province), then along highway 8, direct to Vientian (Laos) by across Keo Nua Border Gate.
• Na Meo border gate in Thanh hoa province
From Vinh Loc in Thanh Hoa Province, along highway 217, across Na Meo Border Gate (Na Meo Commune, Quan Son highland district, Thanh Hoa Province) to Laos.
• Nam Can border gate in Nghe An province
From Dien Chau in Nghe An Province, along highway 7, across Nam Can Border Gate in Nam Can Commune, Ky Son District, Nghe An Province) to Laos.
• Tay Trang border gate in Dien Bien province
From Dien Bien Phu City along highway 279 about 31km, across Tay Trang Border Gate to Phong Xa Ly District, Laos.

From China

• Youyi Guan (or Huu Nghi Quan – English name is Friendship Pass);
Huu Nghi Border Gate is 17km from Lang Son City to the north, 171km from Hanoi to the northeast.
• Hekou (or Lao Cai);
Lao Cai Border Gate is in the north of Lao Cai City. From Lao Cai City, across Ho Kieu Bridge - link Lao Cai Province (Vietnam) with Yunnan Province (China) - to China.
• Dongxing (or Mong Cai).
Mong Cai Border Gate is in the northeast of Quang Ninh Province, 176km from Halong City, 327km from Hanoi. From Halong City, along the highway 18 to Mong Cai Town, and then across the Mong Cai Border Gate to China.