How to keep HHVM running together with PHP-FPM and Varnish, and send some routes in any direction.
Example config:
/etc/hhvm/php.ini

; hhvm specific
hhvm.log.level = Verbose
hhvm.log.always_log_unhandled_exceptions = true
hhvm.log.runtime_error_reporting_level = 8191
hhvm.mysql.typed_results = false

/etc/hhvm/server.ini

pid = /var/run/hhvm/pid
; hhvm specific
hhvm.source_root = /var/www/html/
hhvm.server.ip = 127.0.0.1
hhvm.server.port = 9001
hhvm.server.type = fastcgi
hhvm.server.default_document = index.php
hhvm.log.use_log_file = true
hhvm.log.file = /var/log/hhvm/error.log
hhvm.repo.central.path = /var/run/hhvm/hhvm.hhbc

nginx.conf

map $request_uri $HHVMorFPM {
default                     9001; #HHVM
~^/(incompatible|routes)    9000; #FPM
}

vhost.conf

location ~ \.php$ {
...
fastcgi_pass   127.0.0.1:${HHVMorFPM};
...
        }

Turpentine Varnish extension can be tweaked that you can easily keep other parts in the cache, or pass to backend directly.