Kyle you should install some sideways 2x4 or metal cross bracing under the plywood that extends from wall to wall at or near the front and again half way back of the countertop.
Install marble countertop on the wall.
Measure from the wall to the edge of the cabinets in multiple spots for both the width and length of the countertops.
Make sure to immediately clean up any excessive glue from the marble countertop.
Record both the length and width.
You need a completely solid and stable base for the marble slab.
They are fairly easy to install and you will be very happy with the result.
Lay the marble countertop over the wood vanity to trial fit for shape.
Very rarely will you find walls perfectly square and straight.
It is best to have a professional adjust the fit.
If the countertop is too large or too small contact the manufacture to adjust the fit.
If the fit is too large you can try to cut it to the correct size yourself with a circular saw but you risk splitting the marble.
Apply a wet wipe using alcohol run across all marble surface area before using the adhesive.
Marble bathroom countertops come in various textures and color tones.
Pay attention to any wavy walls or out of square walls because they will effect the installation.
Measure the area in which you want to install the counter top.