You sure this is oak? It looks like parawood, AKA rubberwood, "Malaysian Oak" (Sanding is not a good way to get back to the raw wood that stains need to work successfully. To do this correctly, use a chemical stripper.

You might be able to "glaze" the piece to change the color, slightly. You apply a glaze between coats of finish. The problem is that you may not know what finish is on there now and finish compatibility could be a problem. Another alternative is a toner, finish with color in it. Same problem with compatibility.

"Poly" is just one type, the most common type, of varnish. Same problem with finish compatibility. Almost certainly, what is on there now is not "poly." Poly does not adhere well to other things. If you are not careful, it's likely to just peel off.

So take your chances, or strip and refinish.

Here is a link that might be useful: http://www.popularwoodworking.com/article/glazes_and_glazing_techniques

Thanks for the link.

As it's being oak here is the description.

This solid wood oak ensemble is contemporary and chic in design. As well as sleek geometrical lending an air of no frills but a touch of sophistication of white counter top contrasted and a white basin against natural oak coloring. Comes with mirror and a additional storage cabinet. Ideal for anyone looking for a simple yet elegant look.

Items included: Vanity, Mirror, Sink, 1 Wall Mounted Cabinet, Faucet, P-Trap and Pop-Up Drain, Standard hardware needed for installation.

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A chemical stripper is something I could easily use. I might try it on the back of the vanity where it is finished but would never be visible and see what happens.

Upon doing some research I'm sure it's "Malaysian Oak" which is fine with me in spite of the derisive tone of one of the posters. It's durable, resistant to mold, relatively inexpensive, and renewable. What's not to like? I will testify the vanity is solidly built and not particle board crap. All I need to do is make it not so yellow and I will be very happy with it. I'd much rather have "rubber wood" than some endangered species that is being harvested to extinction.

About Rubberwood

Rubber tree is the common name for the tropical tree Hevea brasiliensis. More than 36 million acres of rubber trees, with approximately 90 trees per acre, grow on plantations in the tropics and subtropics of Southeast Asia and western Africa. The tree has soft wood, high limbs and a large area of bark. It grows to 75 or more feet and can reach a diameter of three feet.

Rubberwood for Wood Products

Because of the dense grain of rubberwood, it does not shrink very much during the kiln drying process. For this reason, it is a useful wood product for making furniture and hardwood flooring. Because of its large size, a single tree can provide a significant amount of lumber for products. It is durable and resistant to mold and bacteria, making it ideal for furniture and flooring. It is also compatible with most adhesives.

Latex

The rubber tree replaced the rubber plant in the early 20th century as the main source of rubber. The liquid latex that comes from the tree contains approximately 30 percent rubber. This rubber can then be coagulated and processed to make rubber products.

Environmentally Friendly

A rubber tree produces latex for approximately 26 to 30 years, at which time the tree is cut down and a new one is planted. The wood is not used for furniture making until it has completed its life of latex production, unlike other trees that are cut for the purpose of producing furniture. Rubberwood furniture is considered environmentally friendly, because the wood otherwise would go to waste after the tree completed its latex production cycle.

Quality

Rubberwood has a beautiful grain that is ideal for making quality furniture. The wood ranges from a pale cream to yellowish brown and is easy to stain, making it a very versatile wood product.

Read more: What Is Rubberwood & Its Quality? : eHow.com http://www.ehow.com/about_6376639_rubberwood-its-quality_.html#ixzz2KPI4AGIL

>Upon doing some research I'm sure it's "Malaysian Oak" which is fine with me in spite of the derisive tone of one of the posters.

Please, the derisive tone was just because I don't like people being lied to by creative marketing folks. How would you feel if you bought a new sweater labelled "100% Merina Wool" and upon further research found out it's 100% polyester? While it may be a fine sweater, it's not what you thought you were paying good money for. And while Merino wood is an identifiable product, Merina wool is whatever someone decides to call it. (Of course, the textile fiber identification act makes this illegal. But it's not illegal to do the same thing for wood or wood finishes).

If you are worried about "some endangered species that is being harvested to extinction," you should avoid most wood products from Asia. Buy USA sourced, where there is more standing timber today than there was 100 years ago.

"Rubberwood has a beautiful grain that is ideal for making quality furniture. The wood ranges from a pale cream to yellowish brown and is easy to stain, making it a very versatile wood product."

I'd say if the wood itself has yellowish tones, then even if you strip and refinish you will still end up with yellowish tone.