What plant is this? It has white /pink flowers growing from the leaf margin. It loves water, and shows an irregular growth habit. I was told it is from Vietnam.
It may have come from Vietnam, but it's not a native of Vietnam. Cactus are native to the New World only. Any cactus on any other continent is a transplant. SE Asia has been growing several of the "jungle" cactus varieties which have big fruit (dragon fruit).
This group of cactus from Central and South America are definitely NOT cold hardy.
Your specimen is quite nice.