I never used that software but I used x264.
The "VBV maximum bitrate" is not the max bitrate for the movie,
is the max bitrate for the VBV (Video Buffer Verifier).
The vbv is used to constrain the output bitrate so it is suitable for
streaming in a bandwidth constrained environment.
As the movie stream is downloaded by the client it's stored in the vbv buffer.
A frame must be fully downloaded into the vbv buffer before it can be decoded.
Commands for your options in x264:
--vbv-bufsize 1500k --vbv-maxrate 1500k --vbv-init 0.9 --ratetol 5.0
sets the size of the vbv buffer in kilobits,
sets the maximum rate the vbv buffer should be assumed to refill at,
sets how full the vbv buffer must be before playback starts,
setting this higher allows vbv to fluctuate more at the risk of possibly
violating the vbv settings (affects vbv aggressiveness).
For movie bitrate there are 3 options in x264:
--bitrate --crf --qp
encode the video in target bitrate mode CBR (the final filesize is known),
constant ratefactor VBR (targets a certain quality),
constant quantizer VBR (targets a certain quantizer).
For full x264 commands and explanations see the link below: