I was unable to turn logging on for mod_rewrite.
I tried setting "LogLevel warn rewrite:info" in my_publish.vhost (see core - Apache HTTP Server Version 2.5 ), but my own rewrites would not get logged. On the other hand, default rewrite rules started getting logged with this setting.
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Mod_rewite is one of the many modules that is installed on apache. You can look at  on enabling logging for rewrite module.
LogLevel alert rewrite:trace6 produces rewrites in error_log. I did not check error_log initially because it was counter-intuitive to search for debug info in errors