Despite how easy it is to fake a CDC card, there have been a number of people who got into the newspaper for getting caught in Hawaii or Canada with a fake vaccination certificate. I don't know how it happened, but they did get caught and faced the consequences.

Overall I think that's a low risk - most people who want to travel will just get vaccinated rather than risk faking a vaccination certificate.

On a separate note, vaccinations massively reduce the odds of getting infected and the risk of spread even if a vaccinated person does get infected. Please stop spreading misinformation that it makes no difference. Even the recent Pfizer randomized controlled trial found that triple vaccinated people were 95% less likely to get infected than double vaccinated people. It doesn't need to be 100% to have a big effect on transmission, particularly when combined with testing.