The problem is that you've defined the same field twice. First as a d:text property and the second is an association.
In your model you've defined
<property name="txm:vacationPerson">
<title>Nom et prénom</title>
<type>d:text</type>
</property>
And as association you've defined:
<associations>
<association name="txm:vacationPerson">
<title>Assignee</title>
<source>
<mandatory>false</mandatory>
<many>false</many>
</source>
<target>
<class>cm:person</class>
<mandatory>false</mandatory>
<many>false</many>
</target>
</association>
</associations>
So it's rendering the first field as text and the second field isn't rendered anymore. Remove the first d:text property or rename it.