I have not found the parts to the Vivaldi RV402 Cello concerto. I am sure they must be out there somewhere.
If you are unable to find the parts there are many ways to make the parts yourself using the score on IMSLP or the score you have.
A) One solution would be to cut and paste each part from the score and make all the individual parts either with paper and scissors.
B) Another solution would be to cut and paste using some program on the computer. Using Photoshop, you could "cut" and "paste" each line of the score onto individual pages.
C) Another solution would be to type in all the notes into a music writing program and make an exact copy of the score and parts could be very easily made from this. There are many programs available that can do this. I personally use Finale on my Mac but there are many others programs and computers that could used as well such as Sibelius and others.
ps. In the mean time, I shall keep my eyes open to find where the parts might have been already published. Most Vivaldi scores, as you probably know, were published by Ricordi. Whereas the parts....?