I've wondered about this. What are travel time frame stipulations usually like?
The forum elders probably have more experience with the technicalities than I do. But the vignette is good for 30 (calendar) days.
Mine is dated from 14 July - 13 August so I have within that window to get into the UK.
Normally, they'll consider the date that you put as your "desired travel date". IE If you got your decision before your "desired travel date", you'll get that date as the start of your vignette.
Mine passed so I got this date, which is actually a week from the day the tracker says that a decision had been made on my visa.