Mixing a lot of biology here.
Vitamin D3 is important for calcium transport and immune activation ( ... T-cells).
Sprycel is a TKI which can (and will) cause myelosuppresson ... low blood counts).
The fact that your blood counts dropped and you had to stop sprycel has nothing to do with vitamin D3. It is a consequence of sprycel. It's actually expected.
Stopping sprycel to allow for counts to rise and then re-starting sprycel (at lower dose) is the proper way forward. You may have to do this repeatedly. It took me 4 times. Start Sprycel, stop, start, stop, start .... and each time on lower dose. I now take 20 mg sprycel and I am "undetected". My blood counts are STILL below normal, but mildly so.
Vitamin D .... is what buys you time.
Vitamin D helps blast cells differentiate - even leukemic ones. In this way, you can stop sprycel so your counts recover and not have a blast crisis (theory).
Over time, vitamin D will strengthen your immune system so that you don't need as much sprycel to keep your CML in check.
That is what I do.