I've used DSL and his child Tinycore. They are good, they even offer a GUI that may be wasteful for simple network troubleshooting, but both lack tcpdump (at least in the downloadable ISO).
Now I've found something just right for the job.
PLD RescueCD: it's reasonably small (89MB ISO), runs entirely from RAM (as low as 40MB), it has tcpdump on board, nc, nmap and a load of other tools.
As a plus, it has no GUI :-)
I've used it to make sure the correct VLAN was mapped on a ESX machine:
Create a custom VM with 1 cpu, 64MB ram, no disk, 1 ethernet adapter bound to the port group you're investigating, and map the virtual CD to the PLD ISO file.
Boot up the VM and you'll be dropped to the root prompt.
Even if ESX does not allow promiscuous mode on VM adapters, you'll still be able to see ARP requests from other machines that are on the same VLAN as your own VM.
That should be enough to understand if you've pinched the right VLAN.
Someone may say that ESX itself shows a network hint next to each adapter, but I've seen that such hint is not always reliable or at least is not real time, so a newly attached VLAN may not be immediately visible.