See if this tip from another Vegas user helps. Save it as a preset in Vegas.
Dialog Norm at -31 (this sets it at unity gain)
RF and Line Mode DRC Off (WYHIWYG)
DC High Pass Filter On (this operates at <=3Hz and takes DC bias out of the mix)
Bandwidth Low Pass Filter On (this takes aliasing noise at >20kHz out of the encode, but it also lowered the rendered volume by -0.1dB in my tests)
90 Deg Phase Shift On (makes it easier for Pro Logic decoding on older equipment, Dolby says it is inaudible in 5.1 except in “very rare” cases)