24-Hour Direct Transit visa for train hanoi to hong kong

I am us passport holder in vietnam. i have a flight from hong kong to america. I would like to take a train from hanoi to nanning and nanning to hong kong. I would like to use the 24-Hour Direct Transit. How would i apply? at the border crossing of vietnam to china? if i wanted to spend a day in nanning could i apply for a temperary stay permit? is staying 72 hours an option?