Does each Frontend app require an S3 bucket and a CloudFront distribution?

Hello.

1. If you have multiple Frontend Web apps with vue.js do you create an S3 bucket & Distribution for each one or do you put them in one S3 bucket?

If you use the same S3 bucket or CloudFront, access logs will be output to the same location and it may take time to search for information when troubleshooting.
Therefore, I think it would be better to separate S3 and CloudFront if they are completely different front ends.

2. How do you handle rollbacks - I'm using a GitHub Actions workflow which on Git Revert will update the contents of the S3 bucket and create an invalidation (this I see as automated rollback) - if it was to be done manually in a scenario where GitHub Actions is down would it be a case of having bucket versioning enabled and restoring a previous version of the bucket?

I think it would be a good idea to enable S3 versioning or take a backup of S3 using AWS Backup.
By doing so, you can also perform rollbacks.
https://docs.aws.amazon.com/aws-backup/latest/devguide/s3-backups.html