Actually, Enterprise Vault.cloud is different than Enterprise Vault with cloud storage.
That being said, Enterprise Vault.cloud utilizes only journaling as the method of capturing archive messages. Therefore, licensing is a subscription-based per mailbox model. Based on the mailboxes that you want to archived (and you do not have to archive every mailbox), you are invoiced annually. Most contracts are 36 months (you can have 24 month and 12 month contracts) and are billed as an annual pre-payment.
Mobile access to Enterprise Vault Personal.cloud is an optional add-on to Enterprise Vault.cloud. The end-user can access the archive through their mobile browser by accessing the URL. The site will determine that it is a mobile browser and will switch you over to the mobile friendly portal for Personal.cloud.
Lastly, you are correct. For Office 365 messaging environments no hardware or software is needed to journal data to Enterprise Vault.cloud for archiving. Directory synchronization is available with Office 365 DirSync and SSO is available using the same ADFS certificate that you use for SSO with Office 365.
Just as Tony (the previous poster) mentioned feel free to reach out to your Symantec sales representative for further assistance and any additional information.