To give you a general overview check out
http://www.tweakheadz.com/how_to_get...with_midi.html
Using Cubase you can assign external midi instruments once set up.
At the moment your midi usb keyboard and a soft synth (vst plugin) is assigned as the input on the left hand side of the cubase screen and you would be assigning the the vst plugin as the output, this is your midi routing. With an acternal pieace of hardware/synth you would be doing exactly the same but the midi output would be assigned to the synth.
EG : Using for eaxmple Cubases Halion
Output : VST Halion
Input : USB midi keyboard
For an access virus
Output : Access Virus
Input :USB midi keyboard
lYou will be fine on the midi BUT the main issue will be routing the sound from the external hardware back into cubase to record and monitor!
At the moment the VST plugins are routed within Cubase so the external audio outputs of your vst plugins are done within cubase and the PC, VIRUTAL. The external hardware however will need to be PHYSICAL so you will have either mono/stereo jacks from the hardware back into the PCs soundcard which will allow you to capture the audio signal back into CUbase to record and playback on your song.
In short you will be fine unless you do not have a soundcard with an appropriate input or a mixer.
Maybe post your kit list and we can help more.