Can I locate a junction box in a wall that will be accessible through a vanity cabinet?
It looks good and accessible. The NEC doesn't say it must be readily accessible only accessible, in case you need to inspect or maintain it. If you are feeling like you want to help another owner or an electrician later on down the road, you might even put a label on the panel saying there is an electrical junction box behind the panel. It could save some time in the future.

Good luck.