Werner, thanks for your report.
Went to the Veun Kham crossing a couple of weeks after you and have a very similar story to report. When I was there, I saw there were 4 Chinese registered cars/campervans looking to cross into Laos. I was thinking these guys are not going to be allowed in. So I went to talk with them, since I'm able to speak some Chinese. I also spoke with Lao customs. They said that while they have no problem with allowing foreign cars/bikes in or out of this border, the decision to allow entry on the Cambodian side rests with the officials there. So they would recommend talking with them first (no need to get your passport stamped) then come back for passport stamping if the OK is given.
I told the Chinese crew that they have 3 options, but only 2 of them are realistic: 1) go down to Phnom Penh for a permit, leaving their cars parked on the Lao side in the meantime. Then return to get your vehicles at the end. Since it was Thursday lunchtime and 537km to Phnom Penh, unless they managed to find a taxi to bring them to PP that day (most likely arriving in PP very late, perhaps after midnight), get the permit on Friday allowing them to be back at the border by Saturday, it would most likely turn into a multi-day affair. 2) speak with the Cambodian officials although it's unlikely they'd be allowed in or 3) Come back next time with a permit. Crossing via Thailand doesn't work as they would need a permit for Thailand. Although some vehicles are making it into Thailand despite the rules, they are not Chinese registered. Chinese need a permit in all cases. In the end they decided it wasn't worth it, said they would look into obtaining a permit via a travel agency in future and they had limited time so they thanked me, turned around and started to make the long journey back to Beijing! Yes, they drove that far!
The consensus seems to be to enter Laos from Cambodia, not the other way round. When I entered Cambodia the following day, I went via O'Smach (as usual). Chong Sa-ngam would have been slightly closer and I've heard they are now letting bikes/cars in this way, but I didn't want to waste time on this and would rather go somewhere I know I won't encounter any issues.
This time, i was told to go to the old customs hut (for the first time ever) but I received a small piece of paper (a customs form) with my vehicle details written on it and a "admitted until" date. I think that's a good move because now you have something to show if you're ever asked. I asked whether I would need to hand in this paper if exiting at another border as that was my intention. Although the official at first seemed to speak quite good English i think he misunderstood me, stating that this paper only needs to be handed back to O'Smach customs, but in the end it became clear I was free to exit at any crossing. I exited at Phsar Prum (Pong Nam Ron, Ban Pakkard) by handing back the paper to customs there. No questions asked, the guy looked at it and said I was done. Perfect.