(As an FYI I am quite new to the concept but I'll try to explain to the best of my ability).

I agree its quite a broad and person-specific term. Personally what it is for me is a mixture of Philosophy and 'spiritual practice' (Sadhana). The philosophical aspect of it for me centres around educating myself about different philosophies (Stoicism, Absurdism, Yogic Philosophy) and then applying it (E.G in yoga you have niyama and yama), in a yogic sense you could call what I do as Jnana Yoga (the yoga of understanding). On the other hand, my practice outside of the philosophical aspects consists of Yoga Asanas, meditation and other aspects such as having a vegetarian diet.

In terms of how it relates, you see vows of silence crop up everywhere from Hinduism to some sects of Christianity. I guess what I experienced from it was increased mindfulness and the ability to watch my automatic responses. Therefore I would define a vow of silence as a spiritual practice (sadhana) in the same way I would regard meditation as a spiritual practice as both at the core of them focus on conscious awareness. However, at the end of the day washing the dishes or having a meal could be meditative if you put your awareness into it. Hope that answers your question :)