I think it’s up to the instructor to choose the warmup, and a grapevine is a standard lateral move in many classes. One would ask her reason for saying it is not acceptable–is she asking because perhaps it is technically not a dance move, or is it that she thinks it’s unsafe? Other reasons?
The instructor has the final say here, in my opinion.
I think the equally important question about grapevines in warm-ups is WHEN it they could be done. I would NOT start out with them first thing.
Although a grapevine is a common move in fitness classes, it requires more coordination than marches, step-touches, double step-touches (basically a grapevine without the foot cross behind), heel taps, etc. It’s also a bigger range of motion to cross over the body.
With the purpose of a warm-up being to transition a person’s body and mind from their outside life into the class, I would argue that a grapevine is a progression, not a base move. I start with marches and hustles. Small range of motion moves. Boring, but our clients are used to moving in the sagittal plane so I start with something they’re already adept at doing. Then step touches to introduce frontal plane movement. Then variations of the step touch like two step touches to the right, then two step touches to the left. Then two step-touches with a bit bigger range of motion (in this case, distance). And finally, grapevines. It’s not like I take four hours preparing for the movement, but it’s definitely not the first thing out of the gate.