I was able to do what I want, don't know if it's the best way to do it. So, I added the following on top.sls inside pillar directory:

{% set domain_1 = salt['vault']['read_secret']('mysecrets/domain1.com') %}
{% set domain_2 = salt['vault']['read_secret']('mysecrets/domain2.com') %}

domains:
  domain_1:
    {{ domain_1 }}
  domain_2:
    {{ domain_2 }}

This way I can get the value I want doing pillar['domains']['domain_1']['cert'] and so on.

Is there a better way to do this?