By default VanitySearch takes 8*(MP number),128 for the grid size.
You can set it up using -g (you need the last release for setting 2D grid size)
ex:
VanitySearch.exe -t 0 -gpu -g 48,256 1TestMe (with a single GPU)
VanitySearch.exe -t 0 -gpu -gpuId 0,1 -g 48,256,64,512 1TestMe (with 2 GPU)

It is good to take the first value as a multiple of the multiprocessor count and the second as a power of 2.
The number of multiprocessor is given with:

VanitySearch.exe -l
GPU #0 GeForce GTX 645 (3x192 cores) (Cap 3.0) (1024.0 MB) (Multiple host threads)

Here 3 is the number of multiprocessor.