Deleting a custom ISO from Vultr doesn't affect instances already created from it
No. Deleting a custom ISO from your Vultr account does not affect any Cloud Compute instance that was previously installed using that ISO. Once the operating system or software has been installed from the ISO, the instance runs independently of the original ISO file.
However, Vultr does not allow you to delete an ISO if it is still attached to an active instance. To remove such an ISO, you must first detach or delete the associated instance. After the ISO is detached and no longer in use, you can safely delete it from your account.
This ensures that running instances remain fully operational even after the original ISO image is removed from your Vultr account.