Why do you need to use both? Or are you saying that they're both equivalent and you'd had success with each?

I've been using gmvault for only one feature: ability to fetch e-mails that match a certain filter.

Reading over the details, gmvault supports two things I don't see in GYB and one thing mbsync doesn't support:

- XOauth

- Sync to another gmail account (incrementally)

Edit: Also supports only backing up emails with a specific gmail tag.