Saving up is not an awful idea, since you can afford more of the upper level powers. This means that at Prime Vampire, you'll have unlocked the ability to select from over half the strengths/weaknesses when spending your points. Ranks go from Fledgling (1-2 Points), Minor (3), Prime (4), Master (5), and Grand Master Vampire (6 Points), though there are some exceptions for Perks that have multiple ranks. Points are tiered based on your rank as a vampire.
You should start with at least 2 points, so you can pick a power right away. Each of these cost Power Points, which are gained by using Vampire Powers and studying the Vampire Lore skill. You'll see a little book, which you can click to bring up a screen that lets you select powers. To pick powers for your vampire, head to the needs tab at the bottom right. This regenerates energy rather effectively, can be done anywhere, and also gives Vampires some experience points toward the next Vampire Rank. When you're low on energy, you can also click your Sim to use the Dark Meditation ability.
When turned into a Vampire, your Sim will by default be able to mesmerize Sims in order to get a drink from them.
