9.) If you have a necromancer tower in your world go to it, dont attack the zombies. Becuase youre undead, the zombies will not attack you. Feel free to murder the necromancers though. Then take their demon book and learn to make zombos then leave.
10.) Your stats are now almost broken. With the bonus of your vamparism which doubles your stats, and if you became a necromancer, another double. Your stats are now quadrupled. Then find a dwarven fortress or a place with LOTS of villagers, then kill about 300 or more. Then go slay a megabeast or two
I don't think these two stand in unmodded game.
Point nine: the zombies won't attack you, unless you are enemy to them, for example because you attacked necromancer. If they don't attack you while being enemies, then it's a bug/glitch. In fact it's easier to become friendly with the necromancers than with zombies after that. This is also true if you come from civilisation which is in war with the particular tower (for example because you previously fought it in fortress mode), then you don't even have to attack anyone as adventurer, they will be hostile, at least zombies.
Point ten: necromancer doesn't give any abilities boost in vanilla game. If you had STR 1300 before becoming vampire, you have effective STR 2600, but it remains at that level if you become necromancer. You can train it, but won't get an instant boost. The only reason to become necromancer at this point is probably to have a reanimate spell, to cover you tracks while retreating or for fun. I don't think that necromancer has anything over vampire in terms of advantages, beyond that spell.
Last time I checked both on the same adventurer it was in v0.44.12 (current version of DF), though the world was created in v0.44.10.