How to apply VST effects to external instrument?

Hi, I have various hardware devices (K2500r, Integra7, VirusTI…) all configured as vst External instruments. The instruments go into the mixer. When I want to record, I route the SINGLE track to the mixer’s subs, and the mixer into the sound card.
The questions:

  1. How can apply vst effects to instrument track of external instruments? ( I can apply a delay or others, but I hear nothing, just the sound without effect.)
  2. Can I apply different vstfx to multichannel external instrument as rack?
    What I need to do this?
    thanks, Antonello
    ps. Virus and Integra7 are connected also via USB but I don’t use their audio output;
    Virus, Integra and K2500 have almost 3 stereo output each;
    I need 9 stereo input audio interface? if so, what is better, audio interface or a mixer like Presonus StudioLive or Soundcraft Signature? Advice/recommendation?

I’ll start with a side note. If you record your synths like you describe, routing them to buses on a hardware mix console, I don’t think there are any benefits to setting them up as External Instruments.

Now, you can absolutely add VSTfx to any channel in Cubase. You can either insert the FX plug-in(s) on the Input Bus (corresponding to the inputs of your audio interface), or on the audio channel of the recorded audio.
To do the latter, open up the Mix Console and locate the insert slots in the racks.
Add an effect to any of the free slots.

I don’t know what this means.

Do you use an external mixer?