Vietnamese mint is known to be as vigorous as the other varieties of mint. That is why I put this one in a pot. I should have known better, as the biggest weed in my front garden is common mint, which escaped from a pot. I put the pot on the soil and even put a piece of plastic to prevent the roots from reaching down into the soil.
For a while, I enjoyed it next to the pond. It was covered in lovely pale pink/white flowers throughout autumn and created a constantly lush look. Then it got bigger, and bigger, and bigger. When I finally had a proper look to see where it's roots were, to say that it had escaped its pot is understating it. It had taken over the pond surrounds, with roots inside the pond (including invading the potted pond plants), and was even popping up between pavers a metre away. So I decided to pull it out. I will dry it out and use it in the fireplace. It seemed easier to remove than common mint. Or am I saying that too early? The reason I decided on complete removal of it was that I rarely use it anyway. And I have pond expansion plans. So, no more vietnamese mint for me.