We believed my son to only be MPI, not MPSI, and as I’m breastfeeding, I cut out all dairy, but not soy. However, we bought Gerber Oatmeal and he couldn’t not keep it down. I just bought the dairy and soy free Beechnut and it stays down much better! Good luck!
(This makes me wonder if there actually is a soy intolerance with my son — maybe he can tolerate it through my breast milk, but not directly?? )