Why is focusing on the breath more difficult than focusing on Mantras, and how important is it before vipassana?
When reciting a mantra you have vitakka and vicāra which helps you get close to the 1st Jhana. But this is also accumulation of Fabrications (more particularly Verbal Fabrications) which should be avoided.

Vitakka and vicāra without the constant awareness of arising and passing inhibits the ability to see impermanence. That is why when you are doing Vipassana you become aware of vibration but not when doing mantra meditation.

To aid you breath meditation you can use vitakka and vicāra to your advantage. Be aware that your are aware of breathing then periodically assess whether you are actually with the breath, but at the reducing frequency. Start with even in breath and out breath and when you notice you can stay a while then do it after each breath and then after few breaths and you your mind just sticks to the task then drop it altogether.

Vitakka and vicara in initial application and sustained application. 1st your bring your mind to the breath then this is the initial application followed by trying to keep the breath in mind.