For a long time, creating custom networks in VmWare Fusion has been a challenge. For some bizarre reason, for years Fusion lacked the network-management GUI commonly found in the equivalent VmWare Workstation product for Windows.
Since switching to OSX last year I've had to edit each VMX file, forcing interfaces to use a custom network definition, but I've recently come across a lovely tool that greatly simplifies this task AND finally clarified for me how Fusion stores network definitions.
This tool is UBER Network Fuser, by Nicholas Weaver. It was released in 2012 when he wasn't working at VmWare yet, and it's not been updated since -- likely because Fusion 6 now offers similar functionality in the twice-as-expensive Professional version (which I think is ridiculous -- Workstation comes in one version with all features, why does the overall-less-powerful OSX equivalent have two?) but it still runs fine on OSX 10.8.5 Mountain Lion.
The tool itself is really easy to use: just go to the Networks tab to add or edit subnets, specifying whether you want to use the Fusion-provided dhcp and NAT services and to run through a virtual adapter. For some reason it won't allow you to customise subnets (you get a random 192.168.x.x net with mask 255.255.255.0), but that can be hacked-in as I'll show you later.
Next, go to the Configuration tab and click on the path to your Virtual Machines (which might already look correct, but won't work until you actually set it yourself). Leave the Preferences Path alone.
Now go to the Virtual Machines tab, refresh the list, select a machine to edit, select the interface you want to assign to your custom network, and select the network. Done!
This is already a nice set of features, but the real kicker is still to come. Close the tool, start Fusion, edit another image, select your network adapter, scroll the option list… your custom network is now an option for all machines! This is because Fusion stores network definitions in /Library/Preferences/VMware Fusion/networking - open it with a text editor and you'll see it contains old and new network definitions. UBERNF manipulates this file, and Fusion is happy to go along with it. You could just change subnet values here, I believe.
Unfortunately Nicholas didn't release any source code for UBERNF (I wouldn't be surprised if some of it ended up in Fusion Professional, to be honest), so there is no way to improve it or even just fix little defects that are probably due to OSX changes between 10.7 / Lion (the version he compiled it on) and 10.8 / Mountain Lion. It would be nice if somebody could write a similar app for Mountain Lion / Mavericks and release it as open-source, or simply charging a few dollars less than the difference in cost between Fusion Standard and Professional. Silly, silly VmWare...