A PaaS cloud service, web or worker role, can be added in a Virtual Network only by changing its service configuration file (ServiceConfiguration.Cloud.cscfg).
You need to add the NetworkConfiguration node just after the Role node as follow:
<?xml version="1.0" encoding="utf-16"?>
<ServiceConfiguration xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" serviceName="MyAzureApplication" osFamily="3" osVersion="*" schemaVersion="2013-03.2.0" xmlns="http://schemas.microsoft.com/ServiceHosting/2008/10/ServiceConfiguration">
<Role name="MyMvcWebRole">
<ConfigurationSettings>
<Setting name="Microsoft.WindowsAzure.Plugins.Diagnostics.ConnectionString" value="DefaultEndpointsProtocol=https;AccountName=xxx;AccountKey=xxx" />
</ConfigurationSettings>
<Instances count="1" />
</Role>
<NetworkConfiguration>
<VirtualNetworkSite name="myazure-vnet"/>
<AddressAssignments>
<InstanceAddress roleName="MyMvcWebRole">
<Subnets>
<Subnet name="MainSubnet"/>
</Subnets>
</InstanceAddress>
</AddressAssignments>
</NetworkConfiguration>
</ServiceConfiguration>