Low serum levels of Vitamin D are common in CLL with a low reading at diagnosis correlating with a poor prognosis. We won't know whether boosting it into normal levels will slow progression until Mayo Clinic release the results of their study, but many of us with low values (self included) find that boosting it into normal levels improves our quality of life, reducing CLL related fatigue.
This Med Page Today article confirms the general understanding that vitamin D3 supplementation is more effective than vitamin D2: medpagetoday.com/PrimaryCar...
US residents please note that if your doctor confirms you are low in vitamin D, due to FDA approvals, they can only write a script for vitamin D2. In this case, just ask your doctor what equivalent over the counter D3 dose you should take and you'll be financially better off.