Kirsten, I'm so sorry that little Luna is having trouble again. I don't know anything about vitamins but I just wanted to chime in and offer up my support.

Right after I first got Max 8 years ago, he was so stressed that he got a bad case of crystals. My then vet told me to give him Hill's C/D for 6 weeks, then Hill's S/D for 6 months, then Preventative for the rest of his life. That worked well until Speckles needed a low protein diet then he got the same as she did.

Since then he will strain to pee every now and then, and as soon as this happens, he's right back on C/D. Now, with Speckles gone, he will remain on C/D for awhile then I'll move him back to Preventative.

Good luck Kirsten and please give those precious babies a kiss from me.
xoxoxo