Ideally, EV should be using it's own dedicated SQL server (if possible). Failing that, as long as the server is spec'd well enough, and not hosting 100's of high-resource instances, a shared SQL server should be ok.
Reporting is just like any other database application, it will have data written to and read from it at differing intervals.
Extract from the EV2007 Installation and Config guide :-
Where and when to install Enterprise Vault Reporting
Typically, the Enterprise Vault Reporting component is installed without any other Enterprise Vault components on a Microsoft SQL Server Reporting Services( SSRS) server, separate from the Enterprise Vault servers. However, if required,
and providing the prerequisites are met, you can include the Reporting component
as part of an Enterprise Vault server installation.
After installing Enterprise Vault Reporting you must run the Enterprise Vault
Reporting Configuration utility to configure Reporting and deploy the reports.
Note: You must only configure Reporting after running the Enterprise Vault
configuration utility successfully on at least one computer in the site on which
Enterprise Vault Services are installed.
Prerequisites for Enterprise Vault Reporting
You can install Enterprise Vault Reporting on any machine with the following
prerequisites. The machine does not require any other Enterprise Vault
prerequisites if you are installing only the Reporting component:
■ Microsoft SQL Server 2000 Reporting Services with SP2; or Microsoft SQL
Server 2005 Reporting Services (SP1 recommended).
■ Both the following versions of Microsoft .NET Framework:
■ Microsoft .NET Framework v2.0.
■ Microsoft .NET Framework v1.1.
Note: Enterprise Vault Reporting requires both versions of .NET Framework
to be installed.
A suitable version of Microsoft .NET Framework v 1.1 is installed automatically
with Windows Server 2003.
If necessary, you can download .NET Framework using the link in the Links
to related software folder on the Enterprise Vault CD-ROM.
■ IIS registered with ASP.NET 1.1 for SQL Server 2000 Reporting Services SP2,
or ASP.NET 2.0 for SQL Server 2005 Reporting Services.
■ A network connection to the computer hosting the Enterprise Vault Directory
database. If you are using FSA Reporting, then Enterprise Vault Reporting
also requires access to the FSA Reporting database.
Using Reporting on a 64-bit machine
If you install Enterprise Vault Reporting on a machine with a 64-bit processor
running the 64-bit version of Microsoft SQL Server 2005 Reporting Services, three
of the Operations Reports covering vault store usage will not be available. To
make these reports available, you need to install and run the 32-bit version of
SQL Server 2005 Reporting Services.
As a prerequisite, the 32-bit version of SQL Server 2005 Reporting Services requires
the 32-bit version of ASP.NET on IIS. To run the 32-bit version of ASP.NET, see
Microsoft Knowledge Base article http://support.microsoft.com/?kbid=894435
