"You can also set limits on some of those VMs to force some of their memory to vswap, if you're okay with the performance trade-off. That way the VMs are still presented the same amount of RAM, but a certain (hopefully unused) portion is swapped to physical disk by the hypervisor."
I wouldn't recommend this. Don't use Limits except you really have a good use case for it ( I can't imagine one for memory ). Swapping is bad for your VM and might create a lot of IO stress on your storage. Use shares instead if you want to prefere some of virtual machines.
The amount of shared pages really depends on your OS, applications and if your are using large page tables.
Do you have this low active value all the time or just sometimes? If it's that low all the time you probably should think about resizing your VMs and put less memory in it. This has to planned very carefully.
I don't know about any document talking about troubleshooting memory issues (you can check this general doc here: http://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.1.pdf ), but usually there are two solutions.
"less v(irutal)Ram, or more p(hyiscal)Ram"