Taking a cat to the vet because it threw up once is just going to be a waste of everyone's time and money.
There's many dozens of benign reasons for a cat throwing up, including it being a behavioral issue. A single occurrence is by no means a reason to worry. It would be like taking a kid to a pediatrician because they sneezed.
When it's a frequent occurrence that you should worry. Once is an event, twice is a coincidence, three times is a pattern.