Current state of vyos-1x smoketests have hardcoded features to test. The feature support is inside the base class BasicInterfaceTest

class BasicInterfaceTest:
    class TestCase(VyOSUnitTestSHIM.TestCase):
        _test_dhcp = False
        _test_ip = False
        _test_mtu = False
        _test_vlan = False
        _test_qinq = False
        _test_ipv6 = False
        _test_ipv6_pd = False
        _test_ipv6_dhcpc6 = False
        _test_mirror = False

And all derived classes need to enable the tests that are supported on this interface type. Adding new feature to a given interface (like vif support in T5237) require manually enabling those tests for the given interface.

It would make much more sense, if we can query the config backend for supported interface options - or in other words - is there a CLI node available that corresponds to set interfaces ethernet <name> vif - if that's the case, _test_vlan = True.

According to @jestabro something similar is already available as there is a packed config dict representing all the XML CLI nodes from vyos-1x.