This is a curious one. This is a piece of the directory I use for videos, input and output files, sorted in date/time order, most recent at the bottom. The bottom one was indeed rendered late last night.
I rendered the shaded file yesterday (8/8/2016) so the above stamp is about right for that render.
This morning I re-rendered it, overwriting with a file of the same name. Vegas asked for a confirmation of the existing file.
But look at the above date. It is not today's. The time looks about right but the date is yesterday's.
What this look like is that Vegas sometimes does what most accounting software does intentionally: pick up the system (computer) date when it is started and not change it afterwards as it rolls over at midnight. Accounting people like that because they can spend e.g. several days entering transactions which all apply to a given (single) day. Also accounting software allows you to manually set the program date to (within limits) anything you specify. But I doubt Vegas is doing this intentionally. I reckon it happens if you overwrite a file of the same name.
Win7-64, Pro 13, latest version.
Moreover, yesterday's render was 4.5GB but this morning's is 3.9GB so the file listing shows the right size. This is doubly confirmed by me uploading both to Vimeo; for yesterday's Vimeo said I didn't have enough weekly allowance left. For this morning's render it accepted it fine, just about.