ROAD
From Cambodia: There is a road crossing at Moc Bai (Tay Ninh province 80km from Saigon) connecting Phnom Penh in Cambodia with Saigon. The new crossing into Cambodia via Chau Doc (245 km from Saigon) is passable either by boat or road.
From Laos: There is a road crossing at Lao Bao in Quang Tri Province, about 160km north of Hue, connecting Savannakhet with Central Vietnam. Another road into Vietnam is via Cau Treo, Ha Tinh Province.
From China: There are 03 roads crossing at Ha Khau (Lao Cai Province), Dong Dang (Lang Son Province) and Mong Cai (Quang Ninh Province.

SEA
There are no normal sea crossings into Vietnam although an increasing number of cruise liners sail into Vietnamese waters. The only other international connection by boat is the Mekong River crossing from Chau Doc to Phnom Penh

TRAIN
There is an international rail connection between China and Vietnam, from Peking via Nanning to Hanoi crossing at Lang Son and from Kunming to Hanoi via Lao Cai.

AIR
Vietnam currently has three international airports: Noi Bai serving Hanoi in the north, Danang International Airport serving the central provinces and Tan Son Nhat in Ho Chi Minh City serving the south.
Available flights to Bangkok, Singapore, Paris, Hong Kong, Seoul, Sydney, Berlin, Jakarta, Manila, Taipei, Guangzhou, Vientiane, Amsterdam, Moscow, Frankfurt, Kuala Lumpur, Los Angeles, Melbourne, Dubai, Osaka, Kaohsiung, and Phnom Penh.

I have not been to vietnam by any of the ways. But wander if the roadtrip from cambodia is dangerous to get into Vietnam?

It is not dangerous at all. U can get into Vietnam by bus or train from Laos, Cam or China. Very easy...

as bkkexpat dangerous, do you mean safety.
Road in SEA not safety at all. But i think it is rather safe to take a bus trip from HCMC to BKK. There are some bus company (or tourist co.) offer the services. Come and check it out.
Good Luck !