I don't think they have a CAPTCHA. CAPTCHAs make the users work, Google does this with their reCAPTCHA. The user has to to free work to help Google with their training of machine learning models. I absolutely hate to do work to increase Google's already outrageous profits and leave the page immediately unless it is very important for me to visit it.

Cloudflare has something called Turnstyle where the browser needs to do work. It's a bit of energy waste, but smooth for the user. Unless their algorithm comes to an incorrect decision and doesn't let you in. Then it's infuriating. For me in Europe that seems to be rare, but I have no idea how well it works in Vietnam.