A floor plan, even a sketch as long as you include measurements, would help a lot.

Definitely do a vanity if you can. One with drawers, as well as a section behind doors (for taller items) would maximize your storage. Our MB is tiny, and I can store a lot in my 24" wide vanity.

Also, if you have enough space within your walls, you can get a 6" or even an 8" deep recessed medicine cabinet. DH uses the hall bathroom (with standard depth med cab), leaving the MB all to me! I have a 6" deep Robern medicine cabinet, and can store a lot in there as well.

You could do a hanging cabinet over the toilet, but given the small space, you don't want to make the room look and feel too crowded. Recessed cabinets, or recessed open shelving creates storage out of "dead" space, without cluttering up the open "air" space in the room.

HTH!

Just in case finding space in the bathroom doesn't work, I had a vanity in my room that also served as a desk for me as I was growing up. Lighted makeup mirror and a drawer devoted to makeup stuff solved the problem for me. I liked it so much that I built a vanity nook into my bedroom in the new house too, rather than add the function to the bathroom, which isn't always the most pleasant place to sit....in fact this is the vanity I used then....it originally was white french provincial which I since faux finished.

The countertop you describe is called a banjo countertop. Try googling that and you'll come up with some good visuals (especially on Houzz). I think a vanity is so much more useful in this situation. If you have room, consider using a frameless kitchen cabinet for extra depth, height and drawer space.

I find that unless you're disciplined, however much space you have in a bathroom vanity will get full of stuff and never quite be organized.

Plus, putting on make-up can be a time-consuming activity. For that reason, I like Annie's suggestion of putting the 'make-up zone' in each girl's private space, out of the bathroom.

For sorting/carrying make-up a small cloth garden or craft tote (the kind with pockets sewn onto the outside and one larger area in the center) can be fabulous.

Sounds good in theory sweeby. With two daughters who are into the latest and greates of make up, I think the best place for make up is in the bathroom.
There are spray on make ups, not to mention the hair spray and perfume spray.
My girls are on their own, but when they spend the weekend, I am glad that the bathroom is an easy clean up.

We have a small bathroom - 5X7
I replaced a vanity with another vanity a few years ago.
And what I learned is the amazing things that a custom cabinet maker can do with a small cabinet. There are all kinds of drawers and the space needed for the plumbing beneath the sink is optimized to a great degree giving more room. in a 32" wide by 22" deep cabinet (and I selected a small undermount sink), there are 6 drawers, 3 on each side of the plumbing. None of the retail cabinets I looked at, either at Home Depot or at more expensive places had anywhere near the drawer space we have. (I was also able to have the counter a little higher since my husband is very tall). And best of all, I found someone Very reasonable, IMO (thanks to a reference here on gardenweb in the kitchen forum). For the vanity plus a cabinet over the toilet, the price was only $625 and that was several years ago. In today's economy, I bet the price hasn't changed much.
Good luck.
p.s.if you find a custom cabinet maker, try to set it up so you can visit his shop. I think there are many amateurs (also because of the economy) where the quality may be lacking. "Let the buyer beware"

I second Annie and Sweeby's suggestion. I have a wall-hung cabinet with a mirror above it in our bedroom for my hair dryer, curling irons and the like. That way I get out of the steamy bathroom for doing my hair. I really like that arrangement. Even if you want to keep the makeup -- with all the attendant little bottles -- in the bathroom, perhaps the hair-related activities could be moved elsewhere?