I've applied for a new passport and will go back to my Can embassy to pick it up in a few days.

My Non-O based on marriage has expired already but my entry stamp is good until the end of October.

I'll make a trip to Vientiane in a month or so to get a new visa, but in the main time do I need to do anything with the entry stamp, which is in the old passport, which I requested to get back?

I've heard that it's even possible to use the still valid visas from the old passport, to enter/exit the country with a new passport, just need to present the old passport with visa at immigration. In my case however there will be no visa from the old passport as it has already expired. Can I just show them my old passport with the entry stamp together with my new passport when I do the border crossing, or do I have to go to immigration to transfer it into a new passport?

On departure from the country you will show both passports and immigration will transfer the entry stamp to the new one and stamp you out of the country in it..