Thought I'd post a Dixie update.

We have her on Purina One Sensitive Systems and things looked better for a bit but then the vomiting resumed. I started to keep a record of the times she ate, drank and vomited. She's a big water drinker, sitting at the bowl for up to 5 minutes each time. She always gets a drink right after eating so...

To make a long story short, she's developed an allergy to chlorine in the water, it also put her through a bought of IBS (the frequent vomiting and empty tummy lowered her natural immunities). We're buying filtered in the jugs for now, until my husband can reinstall my tap filter.
Seems some cats are sensitive to it, I never knew that.

She's doing much better and hasn't thrown up for about 2 weeks.
Also, my two boys are on the POSS and for more good news...
less stinky visits to the box so they're retaining more of the nutrients and they're eating fewer times through the day. Even though they are healthy and everything was normal, their box odors could've drained the stain from the wood furniture!