Here's a pretty nifty bitrate calculator. Althought it's designed for TMPGEnc, it's still valid for Vegas Video.
http://dvd-hq.info/Calculator.html

The most crucial inputs are "Source Length" and "Audio Format".

You say that your video is 1 hour 17 minutes long (excluding the slideshow).
If you use PCM audio (1536 kbit/sec)then your VIDEO bitrate should be as follows:

Maximum 8000 kbit/sec
Average 6250 kbit/sec <--- most inportant number
Minimum 2500 kbit/sec

If you use compressed audio (AC3 at 448 kbit/sec) you can bump up your average and maximum video bitrate by quite a bit...