(an update of an older post, now complete up to vSphere 6.5) Your Linux runs on a VMware VM, but which on which ESXi version? You can see for yourself: run "dmidecode" and look at lines 10, 11 and 12.
ESX 2.5 - BIOS Release Date: 04/21/2004 - Address 0xE8480 - Size 97152 bytes
ESX 3.0 - BIOS Release Date: 04/17/2006 - Address 0xE7C70 - Size 99216 bytes
ESX 3.5 - BIOS Release Date: 01/30/2008 - Address 0xE7910 - Size 100080 bytes
ESX 4 - BIOS Release Date: 08/15/2008 - Address 0xEA6C0 - Size 88384 bytes
ESX 4U1 - BIOS Release Date: 09/22/2009 - Address 0xEA550 - Size 88752 bytes
ESX 4.1 - BIOS Release Date: 10/13/2009 - Address 0xEA2E0 - Size 89376 bytes
ESXi 5 - BIOS Release Date: 01/07/2011 - Address 0xE72C0 - Size 101696 bytes
ESXi 5.1 - BIOS Release Date: 06/22/2012 - Address: 0xEA0C0 - Size: 89920 bytes
ESXi 5.5 - BIOS Release Date: 07/30/2013 - Address: 0xEA050 - Size: 90032 bytes
ESXi 6 - BIOS Release Date: 09/30/2014 - Address: 0xE9A40 - Size: 91584 bytes
ESXi 6.5 - BIOS Release Date: 04/05/2016 - Address: 0xEA580 - Size: 88704 bytesNB These DMI properties are set at boot time. Even if your VM gets live-migrated to a host running a different vSphere version, your VM will keep the values it got from the host it booted on. What you see is the vSphere version of the host your VM booted on. It is the VM power-on that matters, so a guest OS reboot will not regenerate the DMI properties. A guest OS shut down on the other hand will also power off the VM, and the next power-on will regenerate the DMI properties.