Is there any way to ensure you become a Vampire?

I'm going to quote from the UESP:

There are many ways to contract the disease, all of them involve encountering hostile vampires and letting them attack you. However, rather than let them hit you in melee, you should keep your distance and only allow them to cast their draining spell as this is the only attack that can impart the disease. To speed up contraction, repeatedly lose and gain sight of the target vampire so the spell is cast multiple times. The check for giving you the disease is passed only when the draining spell hits you, not over sustained action.

So the trick is to fight Vampires and force them to use their Vampiric Drain spell on you. Exploit Line of Sight for maximum casts.

One place you can go to contract vampirisim is Broken Fang Cave. There you should let the vampire use her Heath Drain spell on you. After you've let the vampire use her spell on you, check on magic, and active effects. Then, it should say Sanguinare Vampiris IF you've contracted the disease.