I would recommend using the following, also VMware recommends the use of PowerCLI, which is PowerShell interface with vSphere-specific additions.
http://communities.vmware.com/community/vmtn/server/vsphere/automationtools/powercli
One of the simplest PowerCLI examples, and something which is actually extremely useful on its own, is the Get-VM cmdlet. Which lists VMs.
PowerCLI can be a bit overwhelming to use, But it can be enhanced with a nice toolset and a library of preconfigured scripts to jump-start your mass changes, which is possible with PowerGUI
http://powergui.org/index.jspa
and then the following script:-
Get-VM VMName | Get-VMGuestNetworkInterface -GuestUser Administrator -GuestPassword -HostUser root -HostPassword | Set-VMGuestNetworkInterface -IPPolicy Static -Ip -Netmask 255.255.252.0 -Gateway -GuestUser Administrator -GuestPassword -HostUser root -HostPassword