I have quite a few vst plugins that emulate old analog gear, which recommend to send a 0dBVU (~-18dbfs rms) signal into them for optimal processing. What about the prochannel compressors, tape emu and console emu? Would it be beneficial or does it not matter?

In theory, 32 bit floating point math should mean it's impossible to "overload" the input of a VST, it would just continue to calculate the correct output based on the input. Whereas analogue hardware has a physical limit of the power supply rails in the hardware and will often start distorting well before the signal reaches that level.
 
This should hold true for level independent VSTs like EQs for example, but for any VST that is level dependent, such as a compressor, it is important to feed it the signal level it is expecting in order for the VST to give the results you're expecting.
 
However...  If a VST has been accurately modelled as an analogue device, the input stage should also accurately model what happens if you try to feed it a signal that is too hot,  ie. it should distort.
 
It is good practice to gain stage everything correctly as if it were all hardware.

Spencer
I have quite a few vst plugins that emulate old analog gear, which recommend to send a 0dBVU (~-18dbfs rms) signal into them for optimal processing. What about the prochannel compressors, tape emu and console emu? Would it be beneficial or does it not matter?

I don't really know what the Pro-Channel modules expect but it's pretty easy to put together an analysis/gain setter that you can insert in between other modules.
To do this you'll need a meter and a gain setter. I have Blue Cat's Gain (free) and a couple different metering plugins, VUMT, BC DP Meter Pro, HorNetVUMeter, etc. Use whatever works best for you.
If you want to drive a Pro-Channel module or your effects rack at a specific level:
1. Create a Pro-Channel FX Chain
2. Insert the BC Gain plugin first
3. Insert a meter after the BC Gain plugin
4. Play the track while monitoring the meter and adjust the gain knob on BC Gain to match the level required by the next plugin.
You can slide the FX Chain to any position in the Pro-Channel rack, pre or post anything so you can check all the I/O points in your signal chain. You also get a channel input trim on audio channels so if the signal going to the first thing in the Pro-Channel rack isn't right, you can adjust the channel trim without using an external plugin.
Blue Cat's Gain is a pretty handy utility.

yeah I have VUMT actually and I use it for my Overtone FC70 comp as well as Nebula obviously but the thought just occured to me that perhaps I'm also supposed to do that with the prochannel stuff and it might make it work better? hoping someone at cakewalk could chime in on this perhaps