This article provides an explanation for why a virtual NIC of a virtual machine remains as flexible even after installing VMware Tools or upgrading the Virtual Hardware version.
Modern Guest Operating Systems such as Windows 2008 do not support this type of adapter and do not provide an option to select flexible when creating a virtual machine. Although legacy Operating System such as Windows 2003 still provides this option, this type of virtual NIC should only be used as a placeholder until you decide which virtual NIC needs to be configured for the virtual machine.
For more information, see Choosing a network adapter for your virtual machine (1001805).
Converting from flexible to VMXNET is an automated process during a VMware Tools installation following a reboot of the virtual machine.
To confirm that the virtual NIC has changed from flexible to another type:
If you choose to use VMXNET2 or VMXNET3, delete the existing virtual NIC of the virtual machine. Then add a new virtual NIC and configure with the desired type such as VMXNET2 or VMXNET3.