Sometimes increasing the bit rate can help -> if you don't have room to create the entire video at the higher bitrate, consider encoding just the credits, and merging files with TMPGenc. (remember, you can go to ~800 meg on a mpg2 file for SVCD - more if using an 80 min blank)

If you're doing SVCD playback using a DVD player, the player itself will interlace and convert 24p video for you... This might be an option if your source is at 24p, or if you want to do IVT. Whilst the software might complain, most DVD players will do SVCDs at 24p. Might or might not work better for your credits, easy enough to test out, but it would definitely give you more room on your SVCD for higher bit rates.

It's kind of a subjective issue, one that's gotten me some disagreement from time to time, but you can also try adding noise (some might prefer blur). Mpg2 at lower bitrates (or wmv, rm, mpg4 etc...) can have problems with really sharp edges, causing what might be called a flicker as the edge jumps up & down. Adding a bit of noise (I would try encoding the credit roll to mjpeg1 with a lower qual setting) sort of forces the encoder to maintain more data then it is currently. Blur can work, as can resizing when it forces Vegas to resample -> dropping the frame size (just for the credits) to 352 x 240, rendering that portion, then importing it as a new take causes Vegas to enlarge the video in a way that's noticably less sharp, but not as fuzzy as a blur IMO.