You'd think, but it turns out that they sell the remaining berths as walk-ups at the standard VN Railways rate (the difference being that these tickets don't come with a goodie-bag or free tea and coffee. The family of three in the two opposite berths are being relatively quiet at the moment though, so I can't complain too much, touch wood!)

I can believe that the very top end company, Victoria, don't do this. But Fanxipan Express certainly do. I'd recommend them in other regards though.

This is going to happen more at quiet times - at busy times (perhaps like July or August) you're more likely to be sharing with similar travellers.

I'd also add that I've had no luck whatsoever booking tickets with an international credit card through the official Vietnam Railways website at dsvn.vn - I've always assumed this is because I'm in Vietnam or something though. I have the same problems with VietJet, and can only pay with my local card.