In a vSphere environment, when from the command line you list the virtual machines files in a virtual machine folder; you find that there are 2 files with an extension vswp. So what are these files?

They are the Swap files associated with your virtual machine. And on a default installation of vSphere there are 2 swap files per virtual machines. They are:

1. VM Swap file:
   VM Swap is created when a virtual machine is powered on and deleted on power off the VM. By default this file is created in the location where the VM configuration file resides. If VMKernel fails to create this file the VM will fail to power on.For additional details read:
   VM Swap file
   KB#2146618
2. VMX Swap File:
   The second vswap file is created with a prefix vmx. This is the swap file for the overhead memory reserved for the VMX process. These VMX Swap files help in reducing contention when memory is over-committed.For additional details read:
   VMX Swap file

Both these swap files are automatically created by the VMKernel (ESXi host).