As long as you are taking in enough calories and carbs, it shouldn't be a problem. I'd focus on mostly fruit juices which are higher in calories and carbs, although it is good to drink at least one glass of vegetable juice daily as well.
During my last 14 day juice feast, I had plenty of energy and it didn't disrupt my exercise routine. I was probably averaging around 6-7 liters of juice per day. 5-6 liters of it was usually something like watermelon juice with lime, and one liter of something like carrot, tomato, zucchini, spinach. I did have other juices as well for variety, but that was typical.
I'd rather eat vegan and look like a tree than eat cow and look like a pig.
SW...225 (Jun 12, 2012)
GW1..175 (Mar 22, 2013)