I am facing the same problem with IPP 7.1. When i set restricted VBR bitrate 4mbps i get output at 5 mbps max and 3mbps avg. but as per the umc manual( and Leonid Kulakov above comment) it should not cross maximum bitrate set(in my case it is 4mbps).
i set the restricted VBR in code as shown below:
m_pvidEncoderParams->rc_mode = RC_RVBR;
m_pvidEncoderParams->m_info.iBitrate = m_dwmaxVBR;
Please let me know if i am missing any other setting to achieve restricted VBR in MPEG2Encoder