勇于 follows the pattern X
于 where X is an adjective and
于 introduces what it applies to:
善于 (shànyú, 'good at'),
乐于 (lèyú, 'willing to'),
敢于 (gǎnyú, 'dare to'). These are all slightly formal — in casual speech you'd just say
敢 (gǎn, 'dare to') instead of
勇于.
