It would make it a bit harder to detect that you are using a VPN, but it shouldn't be hard for the ISP to detect that all your traffic is being sent encrypted to a particular IP address.
They wouldn't be able to see you are using X in particular. There may be very specific timing patterns that the X app or the X web app use when fetching related images, new posts, etc, but this also depends on how you scroll the site, and this seems infeasible to prove beyond any reasonable doubt, and also that any ISP would have logs with that granularity. As the parent said, you should make sure no DNS queries go outside of the VPN, for example.
Now, if you post to X, then it would be clear that you used it, which would be a problem according to the resolution, as long as you were in the country.
They wouldn't be able to see you are using X in particular. There may be very specific timing patterns that the X app or the X web app use when fetching related images, new posts, etc, but this also depends on how you scroll the site, and this seems infeasible to prove beyond any reasonable doubt, and also that any ISP would have logs with that granularity. As the parent said, you should make sure no DNS queries go outside of the VPN, for example.
Now, if you post to X, then it would be clear that you used it, which would be a problem according to the resolution, as long as you were in the country.