This post was planed a LONG TIME, however - due to the fact that I could not get it working back in 2015 (See: https://github.com/freifunk-gluon/gluon/issues/496) it had been delayed. However, as I have the need to get Freifunk working as VM on a central ESXi Server - I tried it again - and it worked! Basic instruction to get it working can be found here in german: https://www.it-kl.eu/2015/08/gluon-x86-unter-vmware/
I got following equipment: ESXi 4.1 Server with multiple NICs.
Start up with getting the right image from your Freifunk Team. I got the gluon-fftr-0.8.4-x86-vmware.vmdk from the Firmware Store of my trusted guys from FFTR (https://github.com/freifunktrier/firmware_store/tree/master/firmware/stable/factory [ Trier 0.8.4 is gluon 2016.1.6-3-g9300421, it's just 2016.1.6 + ee597c6 + Webinterface-color-patches ] )
After that, create a new VM on your VMWare ESXi Server. Choose an user definied configuration, "Other" OS and "Other (32-Bit)" Version. I gave the VM a 1 vCPU and 512 MB of RAM - but you can chose as you like. After that, you need to create two Network Devices / NICs. I chose two E1000 ones, and gave the first Interface the Freifunk LAN and added the second Interface to my common WAN (IMPORTANT!). After that, you're not going to create an HDD.
After the creation of the VM, upload your vmdk file to the folder, edit the VM and chose the downloaded VMDK as Harddrive.
On your ESXi Server, go to Configuration, Network and edit your vSwitch. Allow Promiscuous Mode, MAC Change and Fake Transmissions.
After that, go to Configuration, Software, Advanced Settings - to the "Net" section and change Net.ReversePathFwdCheckPromisc to 1 (thanks to FFS-Roland for helping out! :))
After that, you can start up your VM. You should get DHCP on your FFLAN Port - something like an 192.168.1.x address. Go to 192.168.1.1 via browser, give your Router a name, enable Mesh VPN (important!) and maybe change the bandwidth settings. Save and mail your VPN Key to your Freifunk Team. In some hours, you should be good to go (the Key added to their system - and after a quick reboot of your VM, you should be live :))
Thanks a lot to Draco and Ranlvor from Freifunk Trier for their tremendous help :)!