Hi gylstorff
Be sure to use a multitimbral instance; or have your carrier and modulators feeding into the vocoder on appropriate channels (L/R).  The method varies with different plugins.
TAL Vocoder and Vocov are great.  Orange Vocoder is another one to consider.  It isn’t free but the built-in  synth has presets to get you going straight away.
For one without a built in carrier CM Vocoder is pretty good.  It’s bundled monthly on the cover disc of Computer Music magazine.
Addenda : While not being a carrier/modulator vocoder, Delay Lama sounds great as a throat whistling monk.  Throw the X-Y pad around for speech-like inflections.