How can I use my existing vault that is in Google Drive on my iPhone?
You can’t directly. On iOS, Obsidian vaults can only be in two locations:
On My iPhone/Obsidian/VAULTNAME/
and/or
iCloud Drive/Obsidian/VAULTNAME/
https://help.obsidian.md/sync-notes#Frequently+asked+questions
There are a few third-party apps that should be able to shuffle files between the Google Drive folder and Obsidian’s On My iPhone/Obsidian/VAULTNAME/
folder.
This is the current list pinned on Discord:
These are the five methods currently believed to work for syncing between devices if you’ll be including an iPhone and/or iPad:
• iCloud (NOTE: iCloud with Windows is known to cause file duplication issues as well as data corruption and loss, and the Obsidian devs and community strongly recommend that you avoid this combination.)
• Official Obsidian Sync
• Self-hosted LiveSync plugin
• Remotely Save or Remotely Secure plugin and various cloud services, including Amazon S3 or S3-compatible, Dropbox, OneDrive for personal, and Webdav
• Git with the Working Copy or a-Shell iOS app