Rebecca, your Vanda looks good to me . Perfect foliage, and it seems that your newest leaves are longer than any before, which is a sign that you are doing a lot right.
But I'm surprised when you say that you have a south-facing window - and on the 23rd floor there cannot be much outside casting any shade (?) .How is it that your plant is not showing signs of sunburn ? Do you have some kind of fancy glass , blocking infra-red or ultra violet ? Or do you keep blinds drawn or is your vanda way from the window ?
If not , maybe the answer is temperature. Vandas spike after a temperature drop. I don't know how you will manage that ,but I will tell you what my plants get - and you can check out my recent postings of Vanda Robert Delight, Vanda Pat Delight and a couple of others in the past couple of weeks.I have to grow in a greenhouse, which is set to 17 degrees C as a winter night minimum. On cold nights ( minus 5 is forecast for tonight) it will go down to 15, hopefully no lower. A drop below that , say to 10 would certainly start off a fresh crop of spikes ( I have a lot coming up now from the first cold night of the late summer, when the temperature went down to about 10 - lower than I would want to aim for - due to a heating problem that one night). Its not essential to go that low, and I have not done any proper experiments, but I think the main thing is a drop of say 5 degrees below the normal temperature which the plant has got used to.
Try it for a few nights, then look for a spike within say 10-12 weeks.