It's OK -- wouldn't bother me with the lap belt & tether . . . if she's uncomfortable, though, I would consider switching up sooner.... I myself was OK with my kid using hers like that (it was/is often the only safe enough solution) & while I would prefer the lap belt to be lower in a booster, I just don't worry excessively about it in the vest because the vest does provide a layer under the belt that I think helps reduce some of the more severe abdominal injuries. If the lap+shoulder belt fits better in a booster, then I might use a booster more often, assuming she maintains proper positioning (without increasing driver distraction).
I think it was about a year that my kid was able to fit the size small in a similar way. She was technically not yet heavy enough for the large when I decided the lap belt really was too high. One day the lap belt was suddenly not on the hips anymore, let alone thighs. She barely fit the large circumferentially such that I worried enough to make her lock the belt more (that clickety ratchety locking sound is very rewarding & soothing even if it does nothing but put some rainbow colored goggles on me, LOL) but I believe your kid would fit more nicely in the large than mine
So, I think you've got a bit of time to save toward the large (especially with the lap-only belt: tether should help reduce the force on her abdomen) but she would probably fit into the large fine if you bought it now.
