You can take a Vietnamese-plated motorbike into Cambodia and back. We have heard of people taking Cambodian-plated motorbikes into Vietnam, but have no credible sources on this. The two countries share 5 international road border crossings. They are as follows:
* Bavet / Moc Bai
This is the main border crossing between Vietnam and Cambodia, and buses regularly ply the route. It is the most convenient crossing to go from Ho Chi Minh City to Phnom Penh, and can be done in one big day. It is definitely possible to take a Vietnamese-plated motorbike across the border at this international gate, although you may be asked for fees or bribes.
* Phnom Den / Tinh Bien
This border crossing is close to the Vietnamese town of Chau Doc, and can be a convenient place to get to Kampot, or vice versa. We took our Vietnamese-plated bikes into Cambodia at this border crossing in July, 2014 and had no problems doing so. We were not asked for any additional fees.
* Prek Chak / Xa Xia
Otherwise known as the “Ha Tien” crossing, this coastal border between Vietnam and Cambodia is open to international tourists. We have met people who took their motorbikes across the border at this checkpoint and had no problems.
* Le Thanh / O Yadao
The northern-most border crossing links Ban Lung in Cambodia with Pleiku in Vietnam. There are stories online of people having no problems crossing the border with a Vietnamese-plated motorbike at this checkpoint, but the information is dated.
* Trapaing Sre Border Crossing
This border crossing is close to the town of Snoul in Cambodia and Dong Xoai in Vietnam. This is the one we used to cross the border with our bikes back into Vietnam in October 2014 and had no problems at all.