tourista, guten tag. Where did you hear about the "special stamp" being issued by the Consulate of Vietnam in Sihanoukville? If it was in my old April of 2001 Bayon Pearnik story on
http://www.bayonpearnik.com/html/PDF/Issue56one.PDF
you can forget about it. Probably, the Vietnamese officials who work there are still trying to sell this "special permit" to unsuspecting tourists, but it was always a worthless piece of paper. The story was always and still is good for a laugh. It is only another local scam. If you pay a visit to the Vietnamese officials in Sihanoukville, ask them about it but don't actually buy it. You do not need a "special permit" to cross the Cambodian border with Vietnam no matter which border gate you are using. You need only a standard every day tourist visa.
BTW, even though the Vietnamese consular officials in Phnom Penh and Sihanoukville still sometimes hand out the old visa application forms which ask for your date of entry and your entry point into Vietnam, the new visas being actually issued by the Vietnamese do not list this information. The new visas being issued for Vietnam are valid for any entry gate on any date within something like 30 days of the date of issue of the visa. No worry at all.
If you want to try to take a motorcycle between Cambodia and Vietnam, via the Prek Chak / Xa Xia gateway or elsewhere, your real problem is the motorcycle that you are on. See the thread on
Link removed
In Phnom Penh, the most popular motorcycle today is a 250 cc dirt bike, but you are not going to be able to take this motorcycle across the border into Vietnam. It is nearly impossible to find a 125 cc Minsk in Phnom Penh, because this old Russian motorcycle is not popular in Cambodia, so you are going to need to find something else. A 90 cc Honda Dream step-through is an idea, but these small motorcycles are not good for touring on Cambodia's often terrible roads. The only real option is a 4 stroke 125 cc Honda Degree, the same motorcycle that is often seen pulling small trailers full of people and vegetables around in both Cambodia and Vietnam. Most likely, this is your only hope. Ask about these motorcycles at the shops in Phnom Penh.
Not even mentioned here is the question about legal registration papers and licence plates... Having never bothered with this myself, I can't answer questions about it. You have a lot to do in Phnom Penh if you want to try to figure out about how to take your own motorcycle across the border into Vietnam. The motorcycle shops in Phnom Penh might have information for you.
Lastly, if you haven't been there before, understand that Cambodia's Kampot Province is a very small place. It is only three hours from Phnom Penh to Kampot on Hwy 3, and less than an hour from Kampot to Kampong Trach, the gateway to the Prek Chak / Xa Xia crossing. Even if you are not able to cross at Prek Chak / Xa Xia, it is a simple ride back to Phnom Penh on these main roads. From Phnom Penh, it is less than 2 hours down Hwy 2 to the Phnom Den / Chau Doc crossing, which has been a fully international crossing open to even foreigners on motorcycles for a number of years now. There is also a dry season only road directly between Kampong Trach and Tun Lop on Hwy 2, which is only 15 minutes to the north of the Phnom Den / Chau Doc crossing. The trip takes only two hours or so, but I do not recommend doing this particular road without a local guide, because it really is nothing than ox car tracks. Whatever the case, the point here is that Kampot Province is a very small place. Even if you cannot take your motorcycle across at Prek Chak / Xa Xia, you are only a couple hours away from the Phnom Den / Chau Doc crossing, where you should have no problems at all >>if your motorcycle is less than 175 cc and all your papers are in order.<<
A stay in Kampot itself is worth a couple of days at least, because there are many things to see nearby, including Bokor Mountain and Kep and the new dam project mentioned above. If you make it to the area on a motorcycle, please do upload your trip report here, whether you are able to cross at Prek Chak / Xa Xia or not. Everyone here is interested in such reports. At a minimum, maybe you can take some new pictures of the new buildings on the Cambodia side of the Prek Chak / Kampong Trach road?
Enjoy your visit.