another problem with pgsql and interva/timestamp

Hi posgresql gurus!

As you see, I have a LOT of problems with interval and timestamp
types...

Now, I'm trying to add a numer of days to a date.

If I do this:
select now() + interval '2 day'

I get a datetime two days in the future, thats ok. I want to do this,
but I have these "2" into a variable inside pl/pgsql.

And, when I try to do this:
(N_cantidad_dias is numeric, D_Fecha_hasta is date, v_aux is varchar)

N_cant_dias = 2;
v_aux = to_char(N_cantidad_dias,''999'')||'' day'';
D_Fecha_hasta := now()+ vaux::interval;

I get an error: Cannot cast type character varying to interval.

Then, I try in anoter way:

N_cant_dias = 2;
v_aux = to_char(N_cantidad_dias,''999'')||'' day'';
select into D_fecha_hasta now() + ''''v_aux''''::interval

but I get this error messages:
WARNING: line 110 at select into variables
ERROR: parser: parse error at or near "$1" at character 20

Really, I cant detect what is wrong here...

Thanks in advance!

--
Fernando O. Papa
DBA