Hi Olivier

First things first, SRT is not a set and forget protocol like RTMP.
You need to measure the RTT and know what bandwidth you will be streaming at.
To give a clue you can use ICMP (Ping), I like it precise and use tools from Haivision or iPerf.
It is better to measure latency with UDP traffic.

The absolute minimum you have to consider on your transmitter side are "latency" and "maxbw".
The "latency" is specified in milliseconds, and if you do not specify this information, the default value of 120 ms is used.
Haivision describes the latency setup in several articles. You can find one here: Haivision
The "maxbw" is specified in bytes per second (not bits) and should be set to your maximum streaming bandwidth (example: 3Mbps * 125000 = 375000).

Now to why this was most likely part of your problem.
If you set the correct 'latency' without 'maxbw', it will lead to bandwidth exhaustion.
I am not aware of any way to provide this parameter in vMix.

Reference: Github

The problem can be solved in many different ways, here are some examples:

• Restream a local RTMP stream over SRT with FFMPEG. Example:
  Code:

  ffmpeg.exe -fflags +genpts -listen 1 -re -i rtmp://0.0.0.0/rtmp/srt -acodec copy -vcodec copy -strict -2 -y -f mpegts srt://host:port?mode=caller&latency=latency&maxbw=maxbw

  and then stream from vMix to rtmp://127.0.0.1/rtmp/srt
  
• Use on-premise srt server from todostreaming to receive UDP and restream it with SRT
  
• Use nimble streamer to receive NDI and restream it with SRT
  
• Use a streamer with reasonable SRT support
  
• Use a gateway product from a SaaS provider of your choice

Cheers
Jonas