Hi. I'm a bit confused, have you bought the vanity and are looking to paint it? Is the rest of the bathroom fitted as you could add brass or copper taps for instance with a matching mirror? Accessories will also warm it. I like a little wooden stool in a bathroom which you can add accessories to i.e. A candle, natural sponge, flannel etc which has some texture. A plant on the window sill if you have one and then nice accessories for the sink and either some coloured towels or I like hammam ones. I've found a couple of pics but they may not post as houzz has issues :( These are using deep rich colours which may bring the cabinet towards you a little. A darker top for the vanity would also work

Please see below some inspirations you may find helpful. I would agree with Man About The House in his post above, to get the right vanity unit for your bathroom will be good to see some pictures and the colour schemes.

Have you got an image of the basin and unit?

Sounds great, go for it. Once the bath is the other way around, the room will feel less long and narrow as the floor space will be more square (or more short and wide rectangle really). Keep the floor on the lighter side and paint the walls and ceiling the same light colour and it'll also feel more spacious too. You could paint the built-in cupboard the same/similar colour to the walls so that it disappears visually and doesn't seem like a huge thing facing you when you first go in - get it painted in white undercoat first and see how it feels before deciding on final colour. But I'd definitely do the blue toilet/basin unit if you love that colour - it's a very easy colour to work with and it'll be nice to have something a solid colour. Another option is to go colourful/dark on the walls/ceiling as well as units and embrace the fact that it's a small room where you can be more bold with design choices. Keep tiles the same white as your toilet/bath/sink to keep it simple and then you can change paint colour without worrying about what you've chosen as tiles.