I have been trying to do this myself....
It really depends on what style of music you are making and what kind of sampling you want to achieve.

? Because the sample time is pretty short on the MPCyou are going to be limited by how many VSTi parts you can sample!!! I generally sample VSTi at the very end. I usually sample efx loops, or bass riffs, Or Chord hits, or something not long and extended that can be looped easily in the MPC. If I need complex mellodies I will use the external sound source for that.
I generally get a loop going on my MPC of drums and precussion of 4bars. Then I sync Cubase to my MPC so it stays in time with midi clock. It matches BPM only. The reason I do this is I use and external MIDI module that has BPM based effects, so when I use it live with out a PC, I need the MPC to be the master sync clock source.
Then I build on my loop in Cubase recording all midi parts into CUBASE from my controller keyboard....I record either VSTi or my external module...then I will have midi tracks for each part in cubase which are sent to the MIDI IN on the MPC which are then sent to my External Midi Module. Once I am happy with the part I arm that track on the MPC Record it to the corresponding MPC track I rename the track to the patch on the module....Now that track will play on the module when I disconnect the PC>
Once I have a Virtual Instrument track I like, I will add a few insert efx, use some efx automation (I love that feature in SX) add compression and spice it up to the maximum with heavy Eq. So it sounds like its going to fit with the mpc well (I will always eq again through my external mixer if needed after comming out of the mpc)
Once I am happy with it I will bounce it to an Audio Track within Cubase...then export the sound file....I will then open the file in SoundForge and crop or trim the sound, some times I will change the loop points and see if something cool come out of it...Usually not...
I will then import the Sound into the original project in Cubase into Halion....Assign the sample to a note where I want it...and the same note number that it will eventually end up on within the MPC...I will then draw the notes how I want them or play it with the keyboard...I make sure its working and sounding tight...I might go back into sound forge if its not working and start again.... I then record that midi data onto my MPC and give it a new track....Then I copy the file onto Zip and load it into the mpc...I assign the sample to the note/pad....and now that VSTi sound is rocking with the rest of the track....I often use two copies of the sound so I can have a filtered lower volume version running behind vocals or in the chorus if need be..
I will send all sounds to seperate outs on my Mixer....And may add some dist, delay, reverb once I take it live liven it up a bit...Then i will send the whole mix through a compressor and limiter and often send it back into cubase if I wan't to compose and finalise...
PS: You could easily record your VSTi into your MPC directly from your MPC output then use the internal sample trimming functions. However you should bounce down to audio within cubase...Because the sound of the VSTi is often lower quality and you need the cubase algorithms used at when rendering to audio to get the best quality you can..
Peace,
Madness.
Then I copyt