If I edit the XML associated with a page, with, say, VI, why doesn't the change show up on the display? Is there some sort of time lag, or a need to "reload" the entire application before the changes take effect?
Splunkd is serving up an in-memory copy of the xml view, which is why you are not seeing your file system updates via the UI. In order to do so, you will need to reload these files from disk. There are several ways to do this.
Are you editing it on the DS? In that case it takes a minute or two to deploy. Why are you editing your page with vi, though? Why not just do "Edit source" on the dashboard itself? This will always be instantaneous.