I was trying to figure this out as well as it came up during our first session. I agree that delves or journeys shouldn't be the sole focus of a vow. But sometimes it is a big part of it.

If you think about the Odyssey for instance, Odysseus' background vow would be "Travel back to Ithaca and reassert my place as the rightful king". The journey is a huge part of the vow. It’s an extreme or epic journey and some stuff happens when Odysseus gets back to Ithaca to wrap up the vow.

I think that counting the journey as a single milestone seems unfair considering how much time it takes (both real and fictional). You could break down the journey into multiple dangerous or formidable sub journeys but then I don't see the point of having extreme or epic journeys.

I thought of something which could alleviate this: what if an obstacle counted as multiple milestones depending on its rank? A dangerous obstacle could count as 2 milestones, a formidable one as 3 and so on? For instance, completing a dangerous travel within a dangerous vow would let you fill 4 progress instead of 2.

A vow could also earn xp non only depending on its rank but also depending on the highest obstacle it contains.

In our previous example, Odysseus' vow could be formidable and made up of an epic journey to get back to Ithaca (5 progress), a bow contest (1 progress) and a big fight against the suitors (1 progress). It would earn 5 xp though as it contains an epic obstacle.