Nonono, you got it all wrong. The vampires sleep in coggins during the day. Not coffins. I know, it's a close shave between the letters on the keyboard. Might just have been a typo. In that case, I sympathise.
You also have some pretty prolific things wrong about your procedure here.
1) The horse must be pink, not white. PINK. If you try this with a white horse you'll not succeed, because white horses love vampires and want to have their babies. Vampire horse babies, there we go, but as I said earlier, those are laaame compared to zombie horses!
2) If you run into police or gangsters, that's very good. It means vampires are close. Vampires love gangsters, you see, because sometimes they deal this stuff called cocaine, and it hypes up humans' blood so much and vampires LOVE that. Actually, the policemen are usually gangsters in disguise, and the gangsters you'll see in the graveyard are police in disguise. So be careful when you see policemen. Vamps love policemen too, they're reportedly very tasty.
3) You MUST sit upside down on the horse. I know, it's difficult, but you'll have to practice at this. Get a friend to tie you on upside down when you go for your hunt. Also be careful that your virgin horse isn't a stallion, as it might like it when you're hanging upside down. Try getting a virgin gelding instead. I know those are difficult to get hold of in pink, so you might try a Mastiff mix. I heard they have good colouring. Maria is very good at producing Mastiff mixes (especially quarterbredarabaloosamastiffs), I'm sure she'll sell you one if you ask her.
If you don't sit upside down on the horse then how will you be able to dig the vampire grave open? Be careful to carry a shovel with you at all times. Once the grave is open and the vampire is up you need to call someone to untie you from the horse. Then you can slay the pink vampire.
Good luck!