Umm...
If I read this right, your asking if you need the MS receiver to repeat IR commands to a STB? (theres been a few interpretations I can see).
Not sure what your IMON receiver does, but definately the MS receiver is needed if you want plug in and away you go functionality.
Your best bet is to buy the MS remote set. I know its money, but for $39 you get a MS remote (can always sell it here to someone) AND the all important MS receiver and IR buds.
Also worth remembering that you can make up IR buds yourself and/or buy something similar from Jaycar/Altronics.
The good thing with the MS buds, are nice and small and come with sticky pads so you can stick the IR blaster to the front of your STB.
Simply put, the IR buds repeat an IR signal from the receiver (the USB device) and repeat the signal to another device (STB/TV etc)
If you want to use your IMON remote, then as long as it can use the generic MS receiver, then theres no reason why you cant set the system up to repeat the IR signal to another device.
Now comes the tricky part..hardware aside, there are a few ways on getting the IR receiver to repeat the signal to another device.
if you stick with 'as installed', you will only be able to set the system up to repeat the IR signal if during your TV setup. You select 'yes' to having a set top box, and go through the IR learning process.
This is good if you simply want a STB to work with VMC/MCE.
Now, if you want to do funky stuff (like get other buttons to work and/or macros and/or control different devices through different IR beads (the MS receiver has 2 ports)) then look at HIP and the modified MS receiver driver.
This along with HIP allows you to program the IR receiver and remote buttons as you want, BUT you lose the triple tap functionality (text entry vis the remote).
You can also check out Girder, but as its not free, I didnt venture down that path.
EDIT..just had a thought. If you already have the IMON remote and receiver AND theres provision for IR repeater on your IMON receiver, then HIP may be able to do what you want without having to buy the MS receiver/remote.
You`ll still have to get the IR buds from somewhere (buy them or make them) but the HIP drivers for your IMON im pretty sure should be able to repeat commands. Not sure how this would integrate into VMC though.
For my money Id stick with the standard MS jobby. Lots of people here can testify that its one of the few MS things that 'just works'
Hope all the info helps.