There are mixers. There are VCAs. There are devices that blur the lines between the two.
With both mixers and VCAs, they can work with DC coupled signal (control voltages), audio rate (audio), or both. So it's best to know these specs before you buy something.
VCAs can respond in a linear fashion to control voltage, exponential, or both. It's best to know this before buying one of those.
If you buy a module with a set of VCAs, they will often be normaled to work as a simple mixer. The Intellijel quad VCA works in this fashion.
Some mixers also include a VCAs in each channel. It depends on the mixer: Happy Nerding, Befaco, and others have products that do this.
SIMPLE mixers are usually cheaper than VCAs. So if you just need to mix levels with no external control over the levels, go with a mixer. If you need control over levels, then VCAs are the tool.
Complicated mixers with panning, effects sends, even EQ, are a lot more expensive than VCAs.
Just remember you can mix more than audio with most Eurorack mixers and VCAs. Just be sure to read the manual before you buy. Never assume.