my setup for my vinyl is like this: I have one vinyl player who is connected to a (real crappy) dj mixer (Cortex dmix-600). This mixer has a sound card, i guess, because it can play directly from an iPod or USb-disk. But does this sound card does have an effect on the quality of the vinyl songs? Or is this impossible because this mixer will not digitalize incoming audio-signals like from a vinyl player?
But does this sound card does have an effect on the quality of the vinyl songs?
Yes, the digital to analog converters (the sound card of your mixer) of any device have weight in the quality of the output. Better converters will reproduce the signal more accurately.
However, the difference in quality between cheap and expensive converters is very small. That difference is hard to perceive even for well trained ears. In blind tests, sometimes cheap converters are preferred over very expensive ones!
So there is an impact, but it is minimum. I wouldn't worry too much about it.
Or is this impossible because this mixer will not digitalize incoming audio-signals like from a vinyl player?
With that said, the sound card and the converter only come into play during digital to analog and analog to digital conversion. Assuming that the connection between your turntable and mixer is phono, and the mixer doesn't route the analog audio to the converter, then the sound card won't have any impact whatsoever.
Is your mixer making use of its converter in your setup? I don't know. To know for sure how the routing in your mixer works you'll have to google around and/or check the manual, maybe call customer support. But again, unless you have well trained ears and your hearing environment is optimal (acoustically treated and isolated, good quality monitors which are well positioned, etc) I wouldn't worry about it since you won't be able to perceive the difference at all.