VPE resolves the relative path templates/template.xhtml without any problems since it is relative to the current file.
/templates/template.xhtml is on the other hand absolute and thus it needs to know the root of your webapp to resolve it.
A couple of ways to do that:
1) Edit VPE page settings for the xhtml to set the root webcontent.
or
2) Right click project and click Configure > Enable JSF which will setup WTP on the project
or
3) install Jboss tools maven integration and Maven > Update Project Configuration and it shuold be enabled on its own.
#3 is probably your best option here since you are using Maven.