After a bit of searching and testing, you are correct, the default is E1000. This may be because the VMXNET3 adapter is also used for FT, so VMware may be under the assumption not all guests need FT, so E1000 is the default adapter. With that said, i'm not exactly sure how, or if, you can change it to be the default adapter. One way we get around it, and is the reason I thought it was the default adapter is to create a template which uses the VMXNET3 adapter, therefore any guest deployed off that template will have the VMXNET3 adapter.