I used to think it was a matter of vocal mechanics: once they learned to say the letter “V”, they’d be fine. They might need the occasional reminder, but they’d be fine.
They’re not fine. It’s a Chinglish problem. They’re fine until we get a couple of new kids in the class or they get a Chinese English teacher, and then they all slide back into the “W” sound. We have to review this once a year or so.
I use the Interactive Sagittal Section to show them how their mouth should move for each letter. If you use a webcam, you won’t need this.
Then we go over lists and lists of “V” words. (Think medieval.) I try to practice words that the students would see regularly.
Then we have more medieval word lists.
Finally, to drive the nail in completely, I do some of the level-appropriate activities in this pdf file, to show the students why they can’t mix up the two sounds.
Usually, they’re pretty good for a while after this, even to the point of correcting each other.