The one thing worth knowing before you buy a vanity is that the cabinet underneath matters more than the top you'll be tempted to fall in love with. A beautiful stone counter sitting on a poorly built box will warp, swell, and sag at the hinges within a couple of years, especially in a bathroom that gets real steam every morning. Buy the carcass first. Let the finish be the second decision.
Painted finishes in soft, muted tones hold up the best over time and don't fight with the rest of the room. A warm white, a pale gray, a dusty green, these read differently depending on whether the light coming through the window is the flat gray of a winter morning or the gold wash you get on a summer evening, and that's a good thing, not a flaw. Stained wood in a medium tone is the other safe choice, because it hides water spots and the small dings a bathroom collects without anyone trying. Anything very dark or very glossy shows every drop of toothpaste and every hard water mark by Thursday.
When you're standing in front of one in a shop, or better yet the day it's delivered, open every drawer all the way and look at the runners. Cheap ones bind, drag, or sit crooked within a year. Check that the doors sit flush and don't need you to lift and lower them like you're handling something fragile. Run a hand along the inside back panel for dampness protection if it's a floor standing unit near a shower, because moisture gets into corners nobody thinks to look at until the smell shows up.
A vanity is the wrong choice if your bathroom is small enough that you're already measuring the doorway for elbow room. In a tight space, a wall mounted vanity with open floor underneath will make the whole room read larger and let you mop properly, where a boxy freestanding one just eats the square footage and collects dust in the gap behind it. It's also the wrong call if you're chasing a trend finish, because those date a bathroom faster than almost anything else in the house.
The mistake to avoid is buying the top and the cabinet from two different places without checking the overhang and the sink cutout line up. I've seen a counter delivered a full inch short of the cabinet edge, and there's no fixing that with patience.