javavmwrapper should choose the default version of java depending on DEFAULT_VERSIONS of the build.
A locally present ports tree can have diffrent reasons, but configuration of running programs is not one of them, so javavmwrapper shouldn't use the local ports tree as configuration.
To have a way to still configure the default to something diffrent than the build the configs etc/javavm_opts.conf or etc/javavms could be extended or a new config file created.
javavmwrapper has existed for a long time and changing its behaviour to ignore the ports tree, if present, potentially breaks users who rely on the existing behaviour. I won't be making a change that violates POLA and stops looking at the ports tree.
That said, it would be reasonable to propose a patch where DEFAULT_VERSIONS is looked at if environment variables like JAVA_VERSION, aren't set.