I think this is in getting close but the default path should really be defined by the volume group name.  Please correct me if I'm wrong... cc/ @brandondunne 

If @vg is 'volume_group'
1) LV.create 'lv', @vg, 256.gigabytes 
lv.name => 'lv'
lv.path => '/dev/volume_group/lv'

2) LV.create '/dev/volume_group/lv', @vg, 256.gigabytes 
lv.name => 'lv', 
lv.path => '/dev/volume_group/lv'

If @vg is 'vg'
1) LV.create 'lv', @vg, 256.gigabytes 
lv.name => 'lv'
lv.path => '/dev/vg/lv'

2) LV.create '/dev/vg/lv', @vg, 256.gigabytes 
lv.name => 'lv'
lv.path => '/dev/vg/lv'