Welcome to the Community,
rather than storing snapshots in a separate location, you should think of either resizing the datastore or freeing up disk space by moving some of the VMs to other datastores. Although it is possible to set a working directory (individually for each VM) it adds complexity to the environment and doesn't really solve the root issue of too less free disk space. Also remember that snapshots in VMware are treated as chains, where each chain link is in use, so placing snapshots on e.g. cheaper (but probably slower disks) will most likely decrease the performance of the VMs. (see VMware KB: Understanding virtual machine snapshots in VMware ESXi and ESX )
André