So my character is currently a straight Warrior. I'm only at level 13, but I have been putting points into Health/Stamina and focusing on Smithing, Heavy Armor, Blocking, and One-Handed perks. After becoming a Vampire I thought it might be beneficial to add a magic tree to my bag of tricks. I see that being a Vampire gives one benefits to Illusion magic, but I was thinking Conjuration might be better. What do you guys think? Which magic tree would be a great asset? Or is it not worth it to invest in the magic stat now?

You may not want to invest in one right away, but Illusion gets the most benefit as a Vampire and can help you in a tight spot if you keep it leveled to your enemies. You can easily use conjuration for the same purpose, but it gets no direct benefits from being a Vampire.

I personally don't use illusion so I have no need for it and have not put any perks to it. I think that for a straight warrior character you would want something to complement your battle skills as best as possible. if you're high in heavy armor you're probably a tank and can take a fair amount of damage which to me means that you dont really need flee and calm spells. Call to arms might be good if you use a companion and something like mayhem might be useful as well but I think it's just as easy to use a few perks to conjuration and have your summoned stuff take the enemy aggro while you just run around slashing [censored]. And if you take the perks to summon 2 things and dual casting you can summon two dremora lords for like 6 mins each. It's pretty badass to be running around in full daedric armor with my companion loaded with daedric stuff too and have two summoned dremora lords. Its like an army of daedric asskickers running around.

If you intend to become a Vampire Lord I think it's even less important to have boosts to illusion skills because you can't use any of your regular spells while in VL form anyway.

Also, if you've already taken all your stat boosts to stamina and health, it might be advantageous to avoid boosting your magika and just work on enchanting. Save some perks for when your enchanting is maxed and make armor that allows you to cast a certain school of magic for nothing. My max level khajiit onely has 100 magika but he has 500 health and stamina and it doesn't cost me anything to cast any of my magic.

Restoration is also nice for early level vampires. Since your health, magika, and stamina don't regenerate during the day, it's nice to take Recovery (so you can regen magika during the day until your enchanting is high enough to make good armor) and Respite (so your healing spells with restore stamina as well. As a vampire you are tagged as undead. Therefore, if you take the Necromage perk, your enchantments will have a stronger effect on you.

In all honesty, all this can be achieved by other means. Especially if you have dawnguard. For exaple I don't need Respite or Recovery because I magic has no cost for me and I have necklaces and rings that give me a boost to my regen for all stats.

It's really a matter of how you like to play. Let me know if you have any more questions. I have lot's of experience with being a vampire. Both my characters were vampires at one point and one is a Vampire Lord currently.