So I started making a character to be a vampire a few days ago in preparation for the DLC, I'm planning on going vapire questline with this character & dawnguard with my main. She is a Breton Battlemage & so far I am using heavy armor, one-handed, destruction, illusion, & conjuration (necromancy-focused) the most. is there anything I should change while I'm still low level & what level should I think about turning? What perks & spells should I get? My current equipment is full steel armor with apprentice hood & the mace of Molag Bal as my weapon. Only issues I have so far is her being a bit too squishy still.

Definitely get the Restoration perk Necromage. Also consider getting the Alteration perk Atronach. These two combined with Atronach Stone will make you totally immune to any form of magic based attacks.

How far into the tree is necromage?

Two, maybe three perks in Restoration? Atronach is much further up, but it's worth it to be able to laugh in the faces of all Skyrim's mages.

It would be funny if you didn't need the muffle spell to sneak cause you always float as a Lord.

I get why people take the Necromage perk. It does make a Vampire more powerful, but it is glitch. So you may not feel easy about that. I am not knocking people that do that, but understand that by taking that perk it affects EVERYTHING that buffs your character.

Actually I use Illusion spells like fear & fury to take away pressure when fighting groups, comes in real handy against forsworn.

make sure you get your sneak up and have the companies, theves guild and dark botherhood quest line started so they wont attack you when you turn.

well I got the marked for death word wall already so I don't need the brotherhood anymore.