At its most basic level, cultured marble is just a fortified type of plastic that can be molded to fit any shape required, ranging from tubs to showers to bathroom vanities. This means that any type of tool that can be used to cut plastic can also be used to cut cultured marble. There are multiple hand tools that are readily accessible to the average home owner and are easily handled.
One of the easiest tools to use when cutting cultured marble tops and vanities is an angle grinder. With a four-inch blade, it is small enough to be wielded by hand and used to cut down into the surface of the vanity with a carbide blade attached. You can control the depth by manually holding it as you cut, which requires some level of skill but is still fairly easy to learn on the go. The angle grinder also works well for inside corners where a circular saw cannot reach.
If you are planning on removing an entire section of a vanity top, you could use a circular saw with a carbide blade on it. You can set the plunge depth of the saw according to how deep you need to cut through the cultured marble. This makes it easier than an angle grinder for straight cuts because you can guide the depth without having to worry about it beyond setting the guide, thereby letting you focus on pushing the blade and quickly cutting things.
For hard-to-reach areas or for total demolition of the area around the cultured marble vanity, a reciprocating saw with a four-inch plunge blade tipped with carbide is perfect for the job. It will quickly and efficiently cut through all of the material you need to remove and cut it down to chunks for easy removal. However, there is no such thing as precision removal with a reciprocating saw, so, for this reason, it is usually reserved for demo projects.