I tried the following select:
SELECT (id,name) FROM v_groups vg inner join people2v_groups p2vg on vg.id = p2vg.v_group_id where p2vg.people_id =0;
I suppose your p2vg table has also an id field , in that case , postgres cannot find if the id in the SELECT refers to vg or p2vg.
SELECT
you should use SELECT(vg.id,vg.name) to remove ambiguity
SELECT(vg.id,vg.name)