Arbee,
No experience with Spitfire. I just started tinkering with VSL Percussion within MIR Pro. So far, really good results. However, I tend to do a lot of modern production and sound design, so I often compress the crap out of percussion to make it pump. I've gotten interesting results putting compresion on a sub post-MIR, parallel compresion, etc.--but this is all "modern" oriented. The good thing about VSL, esp now with MIR Pro, is the felxibility it gives you with the dry samples. Very forward looking.
It is a good idea to stick with one platform to an extent. I look at it as my go-to platform, doing the heavy lifting. As always, mixing librries, layering, even adding synth pads (and sordinos!) low in the mix helps you get that harmonic distortion and lushness dificult to attain with samples. Having a couple of libraries is a good idea, but the diversity you are contemplating with VSL can prob do that. Adding a solo string to an ensemble really does wonders. I've even pitched them (solo strings) up a few cents and have gotten nice results. To me Chamber Sordinos is incredible. Listen to the demos. I use them more than the Chambers themselves, but again, layering sordinos is fantastic. The good thing with Orchestral is that you get the sordinos. Nevertheles, you will eventually wnant at least two libraries. With the holiday offer, and the superior quality and felxibility of VSL, it seems like a no-brainer to go with VSL now.
finally, get good at VIP 2! The possibiliies are vast. Other libraries promote things like pronounced legatos ("Hollywood" style) etc., but you can get almost anything you want once you avail yourself of the vast editing features of VIP. The ability to customize matrices, the number of switching modes (try velocity switching horns with horns blare or slot x-fading them for ex), stretching, the APP sequencer etc. humanize, filtering, etc goes WAY further than other hosts.
You are making a considerable purchase. I've been there. Considering it carefully is a must. All I can say is that you can't go wrong wtih VSL, esp with MIR PRo and VIP 2, as long as you are willing to put the time into developing your programming chops. The curve is steep but rewarding. Also make sure that you have a system that can handle it all, though VSL is comparatively lighter than other libraries. To reiterate, with the holiday offer, it seems like a no-brainer given you have the money to invest and the willingness to get good at working with a non- out-of-the-box library (not to say you can't jump in right away and do some great work, but you can go much further. The depth is personally a selling point for me whereas for others, not so much.)
On a side note, I've found the plug Decapitator by Soundtoys to be very helpful in creating harmonic distortion, taking the edge off, coloring (esp when used in parallel). If I could afford a Culture Vulture or a Fatso I would get them but the plug is quite good at emulating. This is just my own personal taste, but the demo is worth trying.
HTH
Mike