Bug 186449

ifconfig(8) utility has an ability to automatically load necessary kernel
module when creating first instance of a new (say, dynamic) interface.
Sometimes it works (e.g. ifconfig crate tap0) and sometimes doesn't
(ifconfig create vmnet0).

Fix: 

This problem is most probably caused by the ifmaybeload() function. It
implicitly derives kernel module name from the interface name by removing
numerical suffix and prepending "if_" prefix. Whereas both tap and vmnet
interfaces share the same if_tap.ko kernel module, and there is no separate
if_vmnet.ko one.

Possible workaround (successfully tested): during installkernel make a
copy (or hardlink) of if_tap.ko under the name if_vmnet.ko, that works
fine. By the way, it makes sense to hardlink tap(4) and vmnet(4) manual
pages too.

Comment: I woudn't dare recommend suggested workaround as an ultimate
solution because of possible unforeseen pitfalls (such as filesystems
incapable of hardlinks etc.).
How-To-Repeat: Boot with a clean GENERIC kernel. Become root. Say "ifconfig vmnet0 create".
You should get "ifconfig: SIOCIFCREATE2: Invalid argument" diagnostics.
At the same time "ifconfig tap0 create" works as expected (as well as
"ifconfig vmnetN create" afterwards, when if_tap.ko is already loaded).