Is Office 365 available by default in Vtiger CRM?

No, Office 365 in Vtiger CRM is not available by default.
To use it, you must download the Office365 Connector extension from the Extension Store.

Using the Vtiger connector, you will be able to synchronize your Contacts and Calendars bi-directionally between Office 365 and Vtiger.
To know more about Office 365 integration, click here.