If i have a vampiric dagger, should i take it in my left or right hand? TTBOMK, it doesn't matter.

Will dagger in either hand affect all the damage or just a part of it? Not quite sure what you mean here, but TTBOMK the hit points a character gains from doing damage with a vampiric weapon only accrue from the damage done by that weapon. He/she won't derive any hit point benefit from damage done by a non-vampiric weapon in the other hand. Does that answer your question?

Do i have to have two daggers to get full effect of lifesteal or will having both weapons with lifesteal give me double lifesteal? See immediately above. As I understand it, if a character wields two vampiric weapons, he/she will gain hit points from damage done by either one, but that's all. He/she won't get any doubled benefits.