NOTION 3 supports VST effects plug-ins and VSTi virtual instruments, and there are a lot of them available . . . I use Kontakt 5 (Native Instruments), all the IK Multimedia VST effects plug-ins and VSTi virtual instruments, as well as AmpliTube 3, Twin 2 (FabFilter Software Instruments), and I ordered MachFive 3 (MOTU) last week and it is scheduled to be delivered today, so I will be able to verify that MachFive 3 works, which it should according to MOTU . . .
Of particular interest, IK Multimedia is having a stellar discount sale until May 31, 2012 on its VSTi virtual instruments, where the Total Workstation 2 Instrument Bundle is on sale and includes at least one FREE Xpansion Tank 2 Multisampled Instrument Collection . . .
Total Workstation 2 Instrument Bundle (IK Multimedia)The Crossgrade price is $149.99 (US), as contrasted to the non-Crossgrade price of $199.99 (US), but if you do not have a qualifying Crossgrade for All policy product, you can purchase one of the IK Multimedia iPad applications (SampleTank for iPad) for $9.99 (US) plus sales tax; register it; and then you qualify for Crossgrade pricing, which is fine with IK Multimedia, since I asked about it in the IK Multimedia FORUM and got an official answer from the IK Multimedia . . .
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NOTE: I have an iPad and iPod touch, and as best as I can determine you only need to purchase the iPad app, since it is registered as part of the iTunes Store purchase, but you should check on it if you do not have an iPad, although I think the strategy works. I use a 2.8-GHz 8-core Mac Pro running Mac OS X 10.7.4 (Lion) for digital music production, including NOTION 3 and DIgital Performer 7.24, VST effects plug-ins, VSTi virtual instruments, and Reason 5 (Propellerhead Software), where NOTION 3 is the foundation of the digital music production system here in the sound isolation studio, which maps to everything being based on and synchronized with the set of NOTION 3 original and cloned scores for a song, and while Reason 5 is not a VSTi virtual instrument, I control it with MIDI instructions exported from NOTION 3, where the Reason 5 generated audio is recorded as soundbites in Digital Performer 7.24 via ReWire, which keeps everything synchronized to the NOTION 3 scores and is the digtal equivalent of the way things were done in the 1950s and 1960s by Phil Spector ("Wall of Sound") and George Martin (Beatles) in carefully designed and orchestrated layers, which for recorded music is what makes it possible (a) to have a bit of FUN with auditory illusions and (b) to make a musical group sound "big" or "larger than life" and curiously is the best way to place a musical group inside the listener's head, which is a variation of the fundamental metaphysical rule devised by one of my teenage mutant friends in high school during the late-1960s, which is that "you have to flip-out to flip-in" . . .
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Miroslav Philharmonik, which is one of the five (5) VSTi virtual instruments in the Total Workstation 2 Instrument Bundle, has a virtual festival of choirs and voices (ensemble, female, male), which includes specific vowels, consonants, and so forth, and there is an Xpansion Tank 2 Vocal Collection . . .
The important thing to understand about using VST effects plug-ins and VSTi virtual instruments with NOTION 3 is that NOTION 3 is a 32-bit application, hence has a 32-bit application workspace, and the practical perspective is that every VST effects plug-in and VSTi virtual instrument is "heavy" with respect to resources, which maps to 25 VSTi virtual instruments being the comfortable upper limit for a single NOTION 3 score, which can include a few VST effects plug-ins, where each VSTi virtual instrument has its own staff . . .
If you need more VSTi virtual instruments for a song, then the strategy is to clone the original NOTION 3 score (a.k.a., project) and use the cloned score to add more VSTi virtual instruments, where you keep perhaps five of the staves constant to serve as references for rhythm, bass, melody, and so forth, which leaves 20 staves for new VSTi virtual instruments . . .
Another part of this strategy is that you record the NOTION 3 generated audio as soundbites in your Digital Audio Workstation (DAW) application via ReWire, which is what I do here in the sound isolation studio, where my DAW application is Digital Performer 7.24 (MOTU) . . .
Done this way, what you do is construct an elaborately instrumented song in
layers, which can map to having as many as 500 to 1,000 VSTi virtual instruments for a song, and while 1,000 VSTi virtual instruments might appear to be a lot of instruments, when the instruments are "sparkled", this maps to what otherwise would be 125 instruments, where "sparkling" an instrument is a technique I developed for placing the individual notes of an instrument in as many as eight (8) different panning locations, with the goal being to be able to put the notes into motion within what I call the "Spherical Sonic Landscapeā¢", as is heard in the following basic rhythm section in the "sparkled" Shaker and Cowbell instruments, which move rapidly from far-right to far-left and then from far-left to far-right, which requires eight (8) staves, where each staff is set to a specific panning location in the NOTION 3 Mixer, which for reference has true stereo panning controls, which is what makes the instrument "sparkling" technique practical, since it is absolutely precise in every respect, which is fabulous . . .

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NOTE: This is done entirely with music notation in NOTION 3, where the NOTION 3 generated audio is recorded as soundbites in Digital Performer 7.24 via ReWire, which is the DAW I use for applying special effects, mixing, and mastering. The instruments are a combination of NOTION 3 bundled instruments, FabFilter Software Instruments VSTi virtual instruments, and IK Multimedia VSTi virtual instruments, and there are approximately 75 instruments due to several of the instruments being "sparkled", which is easy to hear when you listen with studio quality headphones like the SONY MDR-7506 (a personal favorite), but it is mixed using a full-range Kustom studio monitor system, since mixing with headphones or a non-full-range studio monitor system does not work. And as a bit of background, discovering and understanding all this vastly technical information is a lot of somewhat dull and boring work, so I do a bit of yin and yang balancing by composing silly DISCO and Pop songs about ladies underpants, which works for me . . .
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"Tastes Like Anarchy" (The Surf Whammys) -- Basic Rhythm Section -- MP3 (8.8MB, 301-kbps [VBR], approximately 3 minutes and 51 seconds)Fabulous! 
P. S. It takes from two to four hours to "sparkle" an instrument for a 3 minute and 30 second song, but since NOTION 3 has true stereo panning controls and everything is done with music notation, it is
absolutely precise, which is what makes it
practical to do all the
logarithmic geometry, since both loudness and panning are logarithmic, which maps to most of the rules being nonlinear and a bit strange unless you are able easily to visualize curl and to have a bit of FUN with vector calculus in real-time on the fly, really . . .
Curl (wikipedia)Really! 