The thing with V8 is theres no fiber.
The deal with getting enough fruit and veg is that it needs to vary and also needs its fiber to be processed and utilised. One V8 a day could count towards your daily intake but you'd still need 4 other portions which were all different to each other to make your 5-a-day. You can have as many liquid veg/fruit a day as you want, it still only counts as one portion, your body gets rid of the excess and it just doesn't help you aside from the fat soluable things like vitamin A which can end up turning you yellow!
Maybe think about things like cherry tomatos, fresh fruit salads/strawberrys, apple slices (somehow can be more more-ish) having extra chopped vegetables added to things like soups, chilli/bolognaise/other sauces, stir frys...try not to see it as something you have to eat in iscolation but something you eat within another part of a meal. Just make sure its around 80grams and your more likely to be getting a correct portion.