If I'm doing the math correctly:
8 x violins x 4 velocity layers = 32 voices for one note x 2 sections = 64 voices for violins
6 violas x 6 celli x 4 basses x 4 velocity layers = 576 voices for violas/celli/basses
Total for 5 notes: 640 voices
So if two voices overlap for even a second (which happens all the time) that will double that number to 1280 voices.
Not exactly 😉 it is (6 violas + 6 celli + 4 basses) x 4 velocity layers = 64 voices for violas+celli+basses, which together with violins amounts to 128 voices for complete string orchestra.
Perhaps you can try to experiment with increased values in the Audio Engine section of VI PRO's General Settings (I believe Stephane already suggested this). These are also described in the VI PRO manual.