90 day multiple entry query

I'm looking into applying at the embassy/consulate (London UK ) for the 90 day multiple entry visa. The advice on their website is to get the visa before booking any flights but on the form you need to put an arrival date & also a departure date. My question is will the 90 days Visa start when i arrive in Vietnam? And will i have 90 days from that date of arrival?? Or.. will it actually start from those dates that i have to put on the application form??