Quote the variable on the include

tasks/vhosts-combined.yml

@@ -17,4 +17,4 @@
     path: "{{ nginx_vhost_path }}/{{ nginx_vhosts_filename }}"
     state: absent
   when: nginx_vhosts|length == 0
-  notify: reload nginx
+  notify: reload nginx

tasks/vhosts-individual.yml

@@ -23,4 +23,4 @@
   file:
     path: "{{ nginx_vhost_path }}/vhosts.conf"
     state: absent
-  notify: reload nginx
+  notify: reload nginx

tasks/vhosts.yml

@@ -12,4 +12,4 @@
     state: directory
   notify: reload nginx
 
-- include: vhosts-{{ nginx_vhost_style }}.yml
+- include: "vhosts-{{ nginx_vhost_style }}.yml"