CASE not working

I have this query which has a CASE in the middle to give me special results.
The problem is that it doesn't interpret my columns as it should.

Here is the porblem:

siprebi=> SELECT getvencimientosancion(190) AS vence, (SELECT codigo FROM
sanciones  WHERE persona = (SELECT persona FROM usuarios WHERE codigo = 190)
ORDER BY femodif DESC LIMIT 1)  AS sancion_original, CASE WHEN vence>=now()
THEN 1 ELSE 0 END  AS sancionado;
ERROR:  no existe la columna "vence"

(the translation of the error is: column "vence" does not exist).

I don't know what I'm getting wrong here.

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Martín Marqués        | select 'mmarques' || '@' || 'unl.edu.ar'
Centro de Telematica  |  DBA, Programador, Administrador            Universidad Nacional                 del Litoral
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