Connecting with PostgreSQL 9.1 using the Ubuntu machine user and peer authentication method

I want to connect to my local installation of PostgreSQL 9.1 using my machine user (who is vagrant). So, after reading PostgreSQL documentation, I thought I just needed to:

1. Add username map in pg_ident.conf:

# MAPNAME       SYSTEM-USERNAME         PG-USERNAME

vp                       vagrant                                postgres

2. Using the map in pg_hba.conf

# TYPE  DATABASE        USER            ADDRESS                 METHOD

local       all                      all                                                   peer map=vp

But I'm getting the error

sql: FATAL:  Peer authentication failed for user "vagrant"

If I try to connect to my server using psql.

I guess I'm misunderstanding the PostgreSQL manual. But, how could I get what I need? (locally connect with the user vagrant like if it was the postgres user)

Many thanks in advance (and sorry for the cross-posting. I asked this in serverfault too, but I think this is the right place)