Okay, I've been looking at the damage you'll get for teeth and bone spear.
1-5: Teeth (main crowd controll skill in Act 1-3)
7-17: Teeth
18-21: Bone Spear
This gives you 4 + 11 = level 15 teeth (you don't actually get a skill point for reaching level 1), and a 4 point bone spear synergy.
Suppose you get +8 to teeth. Then you would end up at level 23 teeth with a 4 point synergy, so your total (ideal) damage would be 24*33.5*1.6=1286 (!) Your bone spear damage would be negligible though.
On the other hand, suppose you get +8 bone spear. Then you would end up at level 12 bone spear with a 15 point teeth synergy, so your bone spear damage would be 119.5*2.05 = 245 damage per spear. At the same time, your teeth would be reduced to +2, or only 18*21.5*1.6=619.2 total damage.
It appears to me that teeth will clearly be your superior skill, especially when it comes to mobs, but quite possibly also as a boss killer. Since you are planning to get a lot of VIT, I would expect that you should be able to get in close enough to hit 'em with 5 or more teeth per cast, which does more damage than bone spear does. But you'd have to test if that is doable.
You could decide to forego bone spear altogether. This would first of all save you the hassle of getting a +bone spear wand and shrunken head. But you could then also consider using a different synergy to teeth. This may seem suboptimal as it involves getting bone armor as another prereq, so you end up doing less damage, but it means you can start getting synergies before level 18, which could be advantageous at the time you are doing most of your leveling.
As an example, at level 15 you could have:
lvl11 teeth
lvl1 corpse explosion
lvl1 bone armor
lvl1 bone wall
say you have +6 to teeth. Total damage is 18*21.5*1.15=445 at a mana cost of 11,
compared to
lvl 13 teeth
lvlv 1 corpse explosion
again with +6 to teeth, which gives damage 20*25=500 at a mana cost of 12.
Thus, the damage is only slightly less, but the mana cost is better, and since you have fewer teeth you should be able to hit with a larger percentage of them. Also you get a 35 point bone armor, and a bone wall which may be useful to draw in monsters(?)
If you are interested in this, I could work out the optimal skill point / synergy progression. I'd need to know exactly at what levels you expect to obtain your +teeth and +skills items.