You can do that today without waiting for a feature: vMix loads VST3 plugins per input, per bus and on the master. Open the audio settings (gear icon) of the input, bus or master and add the plugin there. For a limiter that keeps you inside a client's level spec in the stream and the recording, two free ones that behave well in vMix:

LoudMax (Thomas Mundt) – a transparent brickwall limiter with just threshold and output ceiling. Put it on the master, set the ceiling to the level the client wants, done.
TDR Limiter 6 GE – more control (soft/hard knee, lookahead, separate peak and loudness stages) if the spec is stricter.
If the spec is in LUFS rather than peak, add Youlean Loudness Meter 2 (free) on the master as well, so you can see integrated loudness while the show runs.

Different story if the problem is the room: you arrive somewhere, the house PA is whatever it is, and your vMix output has to sound right on it. That is not a limiter question but a system-processing one. I build a free tool for exactly that, MixDesk X: a software DriveRack that runs on the vMix PC itself. It takes the vMix master via a virtual audio device, you shoot the room with pink noise and the RTA, the auto EQ flattens the PA, and each output bus has its own crossover, delay alignment, feedback destroyer and limiter. So the stream stays clean inside vMix (VST3), and the hall gets its own corrected, limited feed without a second computer or a hardware rack. macOS and Windows, no cost: adelvo.io/mix-desk-x

Order of attack for your case: VST3 limiter on the master first, that solves the client-level requirement. MixDesk X only if the venue is part of the problem.

Fair point, and I would take a native brickwall too. My guess why it is not there: once you build the limiter, the LUFS meter, the gate and the de-esser are next, and that is exactly the list VST3 was meant to cover. Not a reason against the request, just why I would not wait for it.

Small correction only: the two I named are free, no Waves, no licence server, just a file in the VST3 folder. So until it is built in, that is the zero-cost way to stay inside a client's spec today.