Well that looks horrible, but trimming that is nearly the impossible dream. The vanity should never be where it is in relation to the casing. You would need to either scribe it with a scribing compass/jig or use some pattern tracing jigs to transfer the complex shapes to your new piece.
You'd hold the new piece up next to the jamb, then make a couple critical marks on it... such as top of counter... bottom of counter... top and bottom of fillet moulding and plinth moulding... and then go from there.
You'd measure the width at the top and bottom of anything flat to makle sure there is no taper, or if there is so that you can match it. You'd make those measurements to the inside edge of the door jamb (which will be too wide) and test fit it. then once the trim fits closer, you'd likely want to scribe it again... this time more accurately. Then cut it again and test fit it again. Repeat as necessary. The goal would be for a precise fit so that the edge of the casing can then be caulked and painted.
This is a case of cut twice, or three times... not just once.
I'd see about unscrewing the vanity and sliding it over a 1/2" so a full piece of casing can be used and then add a filler between the top and the wall. A piece of wood painted with wall should be good enough.
Remodelling a 60's home. Initially I was gonna replace the 2-gang faceplate+switch, with a similar face plate. So during backsplash installation I tiled accordingly, including installing a PVC Bullnose Tile Edging Trim in two pieces (above & below 2-gang box) to allow for the 2 gang faceplate to place over them. Later I thought to add a wifi switch for the front and back porch lights (both are 2-gang types.) The new touch wifi switches arrived and they are 1-gang form factor though they have two touch switches each. Looking for ideas on how to install and have a clean finish, short of having to break the tiles, the sheetrock...just to install a 1-gang box.
Secondly, how is the yellow thingy installed? They switches didn't come with instructions.
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