I love both seats, but as an everyday seat, we adore our Indy Plus.
My dd did not seem bothered by the Vivo's shorter seat depth, possibly/probably because it sits so low on the vehicle seat compared to other boosters so she gets some leg support from the vehicle seat (the IP seems to sit lower than many others, as well; it takes up significantly less vertical space/blocks much less of my rear window than most others). I love how the Vivo positions the lapbelt (very low on the thighs, no riding up onto the abdomen), but the IP isn't shabby, either (and I wouldn't worry for a child your dd's size, anyway). The IP wins out with the rigid LATCH
love: this feature, wish more seats had it, though it means sacrificing a center seating position in many cases) and adjustable width head and torso wings, plus a nifty recline mechanism. The Vivo is probably a little more cushy, but my dd's never complained in the IP. The shape of the seat is very comfortable. She has zero problems sleeping on long trips. Oh, another plus-the angle of the shoulderbelt guides on the IP make the shoulderbelt sit more parallel to the shoulder (versus seats like the Vivo and Monterey that seem to place the shoulderbelt more perpendicular to the shoulder). I also like that the shoulderbelt can't really fall out of the belt guide when the child unbuckles and flings the belt back. The shape of the guides and the way they are open enough to allow the belt to slide but closed enough to hold it in place is wonderful.
Also, having a tall girl and a vehicle with no rear headrests, this next part sealed the IP deal for us: the IP is taller than the Vivo, and many other boosters. I've tried her in most boosters out there, except the Monterey, new Parkway, and Frontier in booster mode, but knowing how those seats compare to others we
have tried, I feel safe in saying that the IP is probably on par with the Monterey's height, or a smidge taller (it's definitely taller than the old Parkway and the Vivo, and lots taller than the TurboBooster). The new Parkway and the Frontier in booster mode are the only ones I know of that offer more growing room than the IP.