On MyVestaCP, the real default Nginx templates are usually:
/usr/local/vesta/data/templates/web/nginx/php-fpm/default.tpl/usr/local/vesta/data/templates/web/nginx/php-fpm/default.stpl
/usr/local/vesta/data/templates/web/nginx/php-fpm/default.tpl
/usr/local/vesta/data/templates/web/nginx/php-fpm/default.stpl
So check in the following folder for the template:
/usr/local/vesta/data/templates/web/nginx/php-fpm/