Hong Kong to Vietnam by Train

Hello,
I have a question about the Direct visa option. Is it possible to use it from HK via Guangzhou and Nanning to go to Vietnam. I would like to do it by train instead of flying. Is this been done before?
I will be staying for about 3 days in Hong Kong over the weekend, arriving Saturday morning
Getting a double entry is option as i am arriving in China by train via the Silk Route and flying to Japan before re-entering China.

Thanks in advance,
Dave