If you do ask for a discount I would:
1. email the owner via vrbo,
2. Compliment them on their house (ie: their great pool, gorgeous kitchen, cool games room, etc) and mention how much your kids liked (the pool, the games room, a themed bedroom...). I am always a sucker when the kids like something about the house and am more willing to make a deal.
3.Tell them that it's a little out of your price range, but you thought you'd ask if there was a way to make it affordable and ....
4. Have a number to throw out (maybe figure out the price per night and subtract one night's stay. Ex: House rents for 1400 per week, that's 200.00 per night. Ask if they'd rent it to you for 1200.00)
5. Ask them to call you at their convenience or offer to call them to discuss this possibility and then follow up
6. If they agree to the price, make sure you pay as agreed and on time
7. Make sure you write a thank you on their website or guest book, but don't mention the discount, please! It only upsets those who didn't receive the discount and creates a headache for the homeowner!!
Hope this helps!