Toenail 4 blocks in and your done.

I thought of using L brackets to support the 4 cross blocks instead. Would toenailing blocks be strong enough to hold the weight?

Yes. You could use screws too. If it were my vanity I'd put 2 screws into that existing stud and put a few more into sheetrock anchors. No blocks needed.

Would you be able to link me a picture of sheetrock anchors? I couldn't find any on the homedepot / amazon / google. Thank you!

https://www.amazon.com/Qualihome-Drywall-Hollow-wall-Assortment-Hollow-door/dp/B016HJ0O7Q/ref=sr_1_2?ie=UTF8&qid=1504768951&sr=8-2-spons&keywords=drywall+anchors&psc=1

I'd probably use large 2x6 or 8 blocks screwed into the studs, like I would for mounting grab bars.

I'd do what the Smart Lady recommended in the previous post. Two pieces of 2-by cross blocking, one high, one low. Toe screw them into the left and right studs, and let them in and screw them to the center stud. That way a single length of blocking will span both stud bays.

When toe-screwing, if you think that the thread action of the screws will pull the end of the blocking in too far, then hold a 2" thick spacer block behind the end of the blocking as you toe-screw it into the stud. The thickness of the 2" block and the 1-1/2" thickness of the blocking itself will be equal to the depth of the stud you are toe screwing in to (3-1/2").

I recommend you use coated deck screws for the blocking as well as to hang the vanity off the blocking. Not drywall screws.

Easy way to let the blocking in to the center stud? Set your circular saw blade depth for a 1-1/2" deep cut (or the thickness of the blocking itself). Make a series of horizontal cuts through the center stud, spaced about 1/4" apart. Swinging down, lightly whack the pieces of stud vertically with a hammer and they'll break off like dominoes. Clean up any remaining wood fiber with a chisel.

Our vanity was 50" long. I was working by myself. I cut a 1 x 2 a little shorter than the vanity, leveled it, screwed it to the studs. I used this as support so I could bolt the vanity to the studs.

I was going to remove the 1x2 - but my wife said: "You can't see it". So I left it, and painted the same color as the wall. Just a little more means of support.

Good to see you Mongo! Hope you're doing well. So, in your last paragraph are you doing a sort of modified header?

Hey Smart Lady!

Because the center stud is not completely cut through, I'd refer to it as let-in blocking versus it being a type of header.