If you're hitting page files, then you don't have enough RAM. For best performance your pagefile should be static, not "system controlled".

As long as I have been using Vegas, I have noticed that this PageFile Creep, as you call it. I don't think it is anything new in Vegas 8. DVD Architect also has it. As Steve says, you can sometimes alleviate the problem by adding more RAM, but once you have as much RAM as your O/S can handle, there is nothing more to be gained there. You should obviously limit the RAM used for RAM preview, if you don't need it at that moment, although you have to quit and re-start Vegas for that to take effect.

Speaking of that, my "solution" to the problem is to save the project, quit Vegas, start Vegas again, re-open the project, and continue. This usually gets things back under control. Sometimes a re-boot helps a little more but usually isn't worth the additional time, unless a coffee break is already scheduled.

Finally, every user of Vegas (and DVD Architect which also exhibits these problems) should always monitor their disk light. Obviously this will be flickering when playing video on the timeline, but if you notice it turned full on, rather than flickering, and if it stays on for a long time, even after playback has stopped, you probably have entered the unwelcome "thrashing zone," not unlike the Twilight Zone, and Rod Serling will not be there to rescue you.

[Edit] P.S. Grazie, while I know from your Izotope post that you sometimes do need to edit files on the timeline using programs outside of Vegas, if your current project does not need this capability, I have gotten tremendous relief from this problem by changing the state of the Preference "Close media files when not the active application." I can't remember whether the default (checked) state was better, or un-checked. Change it and see if that helps.