Do you only get the life stealing perk ON a nat 20?

That’s what it says. Although I think it’s supposed to be 3d6 instead of a flat 10. Nice flavor but underwhelming mechanically.

hmmm this would be a sexier pick if it gave the temp hp on a roll of 20 or higher

The SRD says 3d6 but the DMG as published says flat 10.  It was revised I think because 3D6 would become 6d6 due to critical rules, while 10 stays 10.  The same may be why the Vicious weapons only give a flat 7 rather than 2d6.  

Would this work with or instead of the warlocks improved pact weapon?

Can this be stacked with +1, +2, or +3?

Seems like you shouldn't get the healing if the necrotic damage doesn't take (such as through immunities or if the target is undead/construct).

This is not as exciting as i thought

Would have been nice for this to Proc on a crit rather than only on a natural 20 (for Champion Fighters)

I presume this means a 20 on the Die. 

Quote from summussum >>

Can this be stacked with +1, +2, or +3?

Yah, this does kind of look like a ride on effect for a +1 weapon.

Would it be better to allow it to heal the user for half the damage they deal?

Maybe, but consider a class such as the paladin that can choose to add additional damage onto an attack after it hits. If a paladin rolls a 20 using this sword, then adds additional divine smite damage onto the attack, they could be healing substantial amounts.

Fix that by making it weapon + stat damage no additional modifiers, so a 1d10+4 averaging between 9-10 wouldn't be the worst especially during the higher levels.

Its what my dm allows for me

So a Vicious Weapon except it does more damage and you get temporary hit points at the cost of an attunement slot?

yes