I do quite a lot of orchestral composition/production work, often doubling samples with real instruments, sometimes samples only. The big pre for VSL IMO are the Performance Tool options.
With both the VSL large library and with the cheaper, but still quite big 'Opus 1' library you get this performance tool, an addition to the EXS24 sampler or Gigagsampler (not sure of other formats) that gives you enhanced phrasing possibilities.
- real legato playing - the legato tool makes it possible to actually play a sample of an instrument 'going from d to g#' automatically or any other interval; as long as you play or program overlapping notes in a 'performance legato' patch you will hear the correct note transitions. Yes, they sampled all possible intervals up and down, from all possible starting notes, with all instruments, on all dynamics... This finally gets rid of the one of the worst problems with samples - that each sample necesarily has it's own attack since they were recorded as seperate notes, which gives you a good detache or portato performance but very unconvincing legato playing.
- automatic alternating repetition - so when you play a staccato passage on one note, it will automatically alternate between different samples of the same note
- a (more complex to use) tool for ostinatos (not included in Opus 1 I believe)
These tools require a bit of study (not more than one afternoon figuring it out and experimenting a bit) and then they give you very realistic performances.
I have both the big VSL 'Orchestral Cube' library and 'Opus 1' and I find that I use Opus 1 a lot more since it's generally more efficient, it allows me to get a complete orchestra and Altiverb reverb from my G5
while it sounds just as good as the Orchestral Cube. I then add some more specialistic instruments or articulations from the Orchestral Cube. So my advice would be: start with VSL Opus 1 and take it from there.
Oh, True Strike Orchestral Percussion is also nice for percussion but not really necessary if you have VSL.
And get a good reverb with VSL. Altiverb is the best I've heard sofar for realism.