I think V8 is a healthy way to get some of your veg. Just remember what CD said about the salt. I think making your own mix with fresh veg would be even healthier.
I don't know where you got the idea that there isn't fibre (roughage) in V8. It's thick. They put whole veggies in and don't filter away the fibre. It is possible to make fruit juices and veggie juices that have had their fibre filtered out. Think of how thick V8 is compared with apple juice. That's because of the fibre in the V8.
BTW, what I wrote about the fibre content applies to the standard, spicy, low sodium, and organic versions of V8. Not necessarily to the other fruit and/or veggie beverages made under the V8 banner.