The painter says he can get a good even stain on it if he does a pre-stain conditioner.

Your painter isn't a wood finisher. If he suggested usng Minwax and promised to get a 'good even stain' because he's using 'conditioner', then I know for sure he isn't a wood refinisher!! Does he even know anything about Maple?

I've done all types of wood, stains, aniline dyes, etc.

Maple is a very tight grain wood and difficult to get dark. I did one a long time ago in a walnut color, but the results were meh.

(this was back in 2012 when I actually used Minwax products! I now know better)

Anyway, here is a vintage maple hutch that I sanded down to raw wood, used the conditioner, and stained in a walnut.

It was in pretty rough shape when I got it.

took that surface all the way done to raw wood. Is this what your vanity wood looks like?

I used a Dark Walnut on this coat,

and then went over it again w/Jacobean to get this shade. Seal coat, steel wool to fine sand, and a few coats of poly. finished result:

You see the blotchy spots? You saw what I started with. a pristine raw wood surface. I used the conditioner and stain correctly. Yet, the coloring is still a bit uneven. If you like this, fine.

top coat result in natural light.

shelves too. The maple always seems to get a little blotchy w/darker colors. it's just the nature of this species.

If you wanted to stain, you should have picked a diff wood.

Aniline Dye? it's water base and penetrates deeply into the wood grain. it comes in a powder form you mix w/hot water

I had a solid oak french door I wanted black. black stain alone wasn't enough to get it really black so Here's what I started with. The dye

1-2 coats of dye, and it still wasn't black enough. the dye makes a good base for ebony stain

You can see the deeper grain here in the oak and how the dye doesn't quite get all of it.

Gel stain in Ebony did the trick. Then I sealed it all w/ebony Penofin oil. waterproofs it too.

It's as black as can be.

did the oak window trim too. see how deep the black looks?

Do you think your painter knows about these 3 steps and all the products used?

First off, lets see this wood vanity. you've posted one teeny tiny pic of some tile but are wanting advice on things we can't see?

get a spare piece of wood like your vanity so you can practice on it. This painter just wasn't going to use the vanity w/o sampling, was he?