Ruppell's Weavers were common in Ziway when we were there, and I think that's what this is (female).
It is not always easy to tell the weavers in this part of Ethiopia and I am not sure that we can be 100% sure from this single photo.
Ziway has reports of a wide range of species, including Baglafecht, Spectacled and Lesser masked weaver - all of which can immediately be discounted as they have pale eyes and this bird has red/brown eyes.
Therefore of the more likely species there are Village, Southern (or Vitelline) Masked, Little Weaver, and Ruppell's Weaver. They are very similar and I must admit that I cant decide if this is an eclipse male or a female.
Getting a feeling of size with a single photo is difficult. This appears to lack the 'heft' of a Village, but is also too big for Little weaver.
I therefore agree that it is likely to be either a Vitelline of Ruppell's weaver. Ruppell's should have a darker eye and a paler bill than Vitelline, it is also more of a Savannah species that Vitelline. To me, I would sway towards Vitelline, but I have seen both at Lake Ziway and it needs more photos and better notes to say if it was in the reedy lake edge or the thorn-belt that runs down to the shore.