combining a single field in two tables into one column.

i come seeking wisdom. . .

i have two tables, one for subcontractors and another for vendors.  subs 
have a primary key of subnum int4, vendors a primary key of vnum int4.  i 
need to create a query that allows me to see a complete list of these 
numbers in a single column.  for instance, subnum (all begin with 9) has 
values 90001, 90002, 90003 (only three records for the example), and vnum 
(all begin with 2) has values 20001, 20002.  i'd like my query result to 
show:

90001
90002
90003
20001
20002

this does NOT work:  select a.subnum as z, b.vnum as z from subs a, vend b;
NOR does: select a.subnum, b.vnum from subs a, vend b;

by the way, the two tables DO NOT relate to each other (i.e., 90001 does 
not have anything to do with 20001). . .

TIA, jt