It is not possible to create a sub-interface using VAT:
vat# create_subif sw_if_index 1 sub_id 0

{
"retval": -69,
"sw_if_index": 3
}
create_subif error: Subinterface creation failed

The same using CLI:

vpp# sh int
Name Idx State Counter Count
GigabitEthernet0/8/0 1 down
GigabitEthernet0/9/0 2 down
local0 0 down
vpp# create sub-interfaces GigabitEthernet0/8/0 0
GigabitEthernet0/8/0.0
vpp# sh int
Name Idx State Counter Count
GigabitEthernet0/8/0 1 down
GigabitEthernet0/8/0.0 3 down
GigabitEthernet0/9/0 2 down
local0 0 down
vpp#

vpp# sh ver
vpp v16.12-rc0~237-g1a939e8 built by root on DUT1 at Thu Oct 20 10:47:12 CEST 2016