The three international airports in Vietnam are Tan Son Nhat International (SGN) in Ho Chi Minh City, Noi Bai International (HAN) in Hanoi, and Danang International (DAT) in Danang (central Vietnam). Vietnam Airlines has hubs in both Tan Son Nhat and Noi Boi. Most carriers connect to Vietnam's three international airports via Singapore, Bangkok(Thailand), Hong Kong, Taipei (Taiwan), or Seoul (South Korea).
Getting into Town from the Airport -- In Hanoi, taxis queue up just outside the arrivals door and will zip you into town for 230,000 VND to 250,000 VND. Taxis to and from Danang's airport will set you back 60,000 VND. Down south, a ride into Ho Chi Minh City is around 180,000 VND.
The overland route from Phnom Penh to Vietnam is reportedly safe and quite accessible. You can arrange transfers with any of the Phnom Penh travel agencies, the best of which is budget Capitol Tour, #14 Rd. 182, Phnom Penh (tel. 023/217-627), which cooperates with the Vietnamese budget-cafe tour operator Sinh Café to make for a relatively fluid connection between the Cambodian capital and Ho Chi Minh City. Note: You must have a prearranged Vietnamese visa when entering Vietnam (visa on arrival is available in the other direction, from Vietnam to Cambodia). Buses leave from the Capitol Tour office in the early morning, arriving in Ho Chi Minh in the midafternoon, depending on the efficiency of the connection. (Note that you'll have to lug your own bags through the long border checkpoint here.)
You can do this same trip by rented car with driver, but you'll have to make separate arrangements on either side of the border, since vehicles cannot cross.
From Laos -- I wouldn't really recommend this long overnight road trip from Vientiane or Savannakhet. You're dropped off smack in the center of Vietnam, at Dong Ha Province just north of Hue (makes for a more limited itinerary or backtracking). It's better to fly from Vientiane to Hanoi or Ho Chi Minh City on Vietnam Airlines or Laos Aviation. Note that any connections with the city of Luang Prabang must go through the Laos capital, Vientiane.
There are regular connections between Vietnam and China at the border areas of Lang Son and Lao Cai, both in the far north. Note that you need prearranged visas for entrance into China and Vietnam, so be sure to plan ahead if traveling in either direction. Trains do not make direct connections to both border points (Lao Cai is far more efficient); you must take short taxi/motorbike taxi rides on either side of the border to get to public transport.
You can cross to Vietnam by boat from a port near the Cambodian capital of Phnom Penh to Chau Doc, a small Vietnamese border town in the Mekong Delta. Contact budget Capitol Tour in Cambodia for connections.
A unique new option is the weeklong cruise from Angkor Wat all the way to Can Tho or My Tho on one of the luxury, shallow draft Pandaw Cruise Boats (www.pandaw.com). Shared rooms on the vessel start at $2,069 for the 1-week duration.