Take absolutely no notice of the old wives tales you are hearing that playing the cello will somehow damage your violin playing. There are a number of professional violinists around who play the cello to an extremely high standard. It is odd however that there are few cellists who subsequently taken up the violin - perhaps because they couldn't face the contortion act violinsts need to achieve in order to play the instrument.
I took up the cello while studying violin at music college and nobody complained or gave me grief about it. I didn't find the fingering or bow hold confusing at all and I suppose it took me about 18 months in total to reach somewhere around DipABRSM standard. I didn't bother taking any exams because I didn't see the point in putting myself through the stress of it while at music college, but it is not beyond the realms of possibility to play both violin and cello to the same standard. As a professional violinist I simply no longer have the time to play the cello as much I would like. In fact, I haven't touched the instrument for nearly a year

Having made positive comments about violinists playing the cello, there are a few salutary points I would add:
Do not take up the cello unless your violin technique is absolutely correct and rock-solid. You won't do your playing or progress on either instrument any favours if you are still shakey with one technique while trying to learn a totally different bowed string instrument.
The most common habit violinists have when they start playing the cello is a rather violinistic left hand, ie. it tends to want to tilt back (upwards) towards the scroll, almost as if you were playing the violin. The hand must remain level, parallel with and over the fingerboard in order to reach the notes correctly and not get hand-strain.
Playing the cello requires significantly more strength in your hands, both left and right, than playing the violin. The left hand fingers will feel weak to begin with and the fingertips can become quite sore. Practice only for short periods initially, unless you want cramp in your hands!
Find yourself a
good teacher - and one that isn't prejudiced against teaching the cello to violinists.
Do make sure you have a comfortable stool or chair and of a suitable height to sit on when playing. Make sure the endpin of the cello is set at a height that allows you to sit with your back straight - don't slump over the cello as if you are pinning it to the ground!! The cello should not be upright when you are playing, but at an angle, so that the lower bouts are between your knees.
Back problems are the biggest issue for cellists - usually a result of bad posture on bad seating. Sitting on chairs where the seat slopes backwards is a no-no. If necessary use blocks under the back legs of the chair to tilt the seat forward slightly.