@Matte Fascinating: When I compare the 唷 vs 优 even the Zhuyin looks different. The Zhuyin on 唷 looks like it should have the same ending vowel as
bo1 and without the 'oo' sound. I'm still not sure how that would be pronounced (
yuhhh?) but don't think I would go wrong pronouncing it the same as
you1. To your question of why it's omitted, perhaps because it only comes up in particles suggests that it's dialectical or regional rather than standard.
Is it actually supposed to be
you1? That's how I hear it said anyway. In fact, I don't think I have heard of anyone say
yo1 or even know how to pronounce
yo1. Is it like
bo1 but with a
+1 for the right variants, as well as a comprehensive collection of alternatives and why they should not be used.