Built for the parents, the master bathroom needs a vanity with sink that is fully-featured.
Meanwhile, treating the master bathroom should be obviously different. The features have to be complete and the design of the vanity has to be proper enough that matches with the age of the parents. Talking about features of master bathroom vanities, more or less, there are two basic needs that MUST be complied with.
The first one is cleaning. The second one keeping things.
Pay attention also to the style of the sink vanity. The best models of vanity for matures or elderly people have to be able to show elegant and warm nuances but still look calm and comforting. A wooden cabinet vanity with an under-mount sink is worth the first priority.
For the guest room, choose vanity that has just basic features but large enough to be completed with two sinks.
What guests need in the bathroom commonly is not that much, compared to what the homeowner want and need in the bathroom. You know, a guest may stay overnight for one or a few days. But, to feel every guest like at home, it is better if is completed with some drawers sufficient to store a few things. Some minimalist sink vanity with two sinks is a good choice. Why should be with two sinks? Well, if there is a couple with their little kids staying overnight, that is going to be necessary.
Some ideas of sink vanity are specifically connected to where the vanities are supposed to be located in, because, sometimes, inside a house or some public building, there are more than one bathroom built for some different purposes, e.g. guest bathroom, master bathroom, and things. Also, there may be a powder room that surely requires the presence of some vanity.