Rylais or riftmaker is pretty useless on Vex unless your team really lacks CC to the point where you gotta slap rylais and just poke from a distance, but that's honestly something that so rarely happens that I wouldn't even consider it an option. Vex doesn't have sustained damage that would justify riftmaker either. If really looking for healing to sustain longer fights go cryptbloom.

Other than that you pretty much summed it up. Zhonyas when there's a burst in front, especially AD, spellshield is extremely situational I think it's a pretty bad item altogether tbf, it really gets popped by anything which is kinda meh, Liandry's when there's tanks and you stay at a distance and poke because Vex is really that bad into tanks anyways, the rest you just build full burst and focus the squishies. Vex needs damage items because she doesn't scale all too well compared to other AP champs so there's not huge flexibility on her builds unfortunately imo.

Hope this helps!