On my screen, the counters look lavender so it would not look good to have lavender walls as well. I'd stick to your original idea of white walls and grey cabinets. I don't think the frame would be a really big issue and if you don't like it once its done, just use the leftover paint from the cabinet to paint the frame grey. It would take less than five minutes to do. Making the vanity functional depends on how you want to use it. Without much information on that, I can't say much but suggest you get drawer dividers for the little things on the vanity and give each kid their own drawer. Any form of organisation, holders, dividers, etc. would make the space work better for everyone.

Agree with the above, plus you can put a trash can under that makeup vanity, or a bucket for bath toys, depending on the kids' ages.

What color are the counters ? From the looks of them on my monitor I'm not sure gray is the right color for them ...idk.

Here are a couple of more pictures of the counters. The medicine cabinet mounted on the wall was the gray color I was going to go with for the cabinets under the sink and for the vanity. I feel I definitely need a wider mirror, maybe with a frame, even though it's a single sink.