They are supposed to honour the date you put, but they don't always.
Lately they have been making the vignette valid from 7 days before the date (i.e. 8th June).
The best thing to do is make it VERY clear on the application form and in a cover letter that you want the visa to start on 15th June... as in, repeat it, bold it, highlight it etc.
Having said that, you should NOT book any flights until after you have received the visa and have checked it for errors. You don't want to go ahead and book anything and then find that either your visa hasn't been issued yet by that date, or it is refused, or it has different dates in it, which means you either cannot travel at all, or you lose money because you have to change flight dates etc.
