This usually depends on the preset you start from. Try modifying one of the given internet presets as a start point. I used MainConcept AVC with bitrates up to 100 Mbit/s. Though meanwhile I use Vegas2HandBrake instead (see my last post).
That said, doing the H.264 render via HandBrake would allow you to use much lower bit rates at much better quality as either Sony AVC or MainConcept AVC would do.
If you don’t yet use a Magix branded version of Vegas Pro and not yet the latest version of HandBrake you could try to use my https://www.vegasvideo.de/vegas-2-handbrake-en“>Vegas2HandBrake workflow which allows to send the Vegas timeline to HandBrake with only one mouse-klick.