Vegas has virtually unlimited tracks, so that's never an issue. I regularly drop loads of video and full multitrack audio on the timeline no problem.
As for the real-time thing, Vegas is insane for that, some people familiar with other editors can't believe their eyes when they see that. Basically, you can be playing the audio (hearing it real time off the timeline), WHILE you drag or expand/contract audio or video events/tracks. So, basically, you can zoom way into the timeline, and move the video event a frame forward or backward (or move the audio for that matter), and literally listen to how the changes affect the sound as you do it. It can be helpful to just loop on a small region as you're doing things like that. It makes it SOOOO much easier to manually sync.
Also, I use Plural Eyes to great success in Vegas, that tool ROCKS, especially for someone like myself shooting on a 12-minute-limited Canon 5D2 where I end up with loads of clips to be synced. There are a few options you can use to help it make sync if it struggles, and I have yet to not be able to sync something with those tweaks.
Finally, I find all versions of Vegas 9x to be somewhat unstable (at least in my workflow, including the new 9d), but Vegas 8c is rock solid stable for me on a Win 7 i7 box. Another thing is that it's helpful to have your audio at 48hz to sync to hopefully the ambient track of your video (which will undoubtedly be at 48). As far as "replacing" the video's audio, no need in Vegas, just mute that track once you have sync and leave it right there in case you need it later.