I've got a few Hikvision IPC-D120 network cameras (H264) and like to use these in vMix.
I entered the rtsp url in vMix and it works but with seconds delay and a lot of freeze moments (even with a buffer).
Why is this not working stable in vMix?

I have used this camera for a year with BlueIris (video security software) and it works super stable with almost no latency.
I also tried H264Webcam and IPCam on my phone, it's also very stable with very low latency.

Is there a difference how these kind of software works relative to vMix?

I didn't use 2 programs at the same time. I was only wondering why it works so much better in other software.
I know that in these other software you have to choose the IP address and also the right camera type otherwise it doesn't work.
(there is a list with hundreds of camera brands and different types for each brand)

It's a pity that I have to use workarounds again, vMix should handle this correct, but I will give it a try.

I had the same problem with my Sony SNC-ZP550.
Less freezes but still seconds delay and after a freeze some more delay.
I never could get audio and video in sync for a longer period.
VLC is stable but still big latency.

Btw
BlueIris is no video mixer of course :)
Although I used it before vMix appear, it had native http jpg stream so I could use my Sony SNC-RZ cameras without capture device.

I suddenly realize that BlueIris might use also http jpeg streaming for the Hikvision cameras instead of rtsp H264.
Can that explain the lower latency?

At my work we actually had to investigate the same issue in combination with our own software (that was actually a hardware decoder, so not vMix). We found out that the problem was that the Hikvision cameras periodically (once every few seconds) send out UDP packets of a different stream type. These UDP-packets are sent to the same address/port as the H.264 packets, which confused our decoder. When asked, we got the reply from Hikvision that these packets were proprietary packets used by the web-interface of the camera. VLC-player handles them properly by the way.

The bottom line: we had to change our software to ignore UDP-packets not belonging to one of the stream type advertised in the RTSP-SDP-info. I don't know anything about vMix internals, but it is possible that vMix also does not ignore these packets and gets confused by them as well. But that's just a guess... It would explain why vMix handles all other cameras perfectly, because I've never seen a camera that does this before either.