Hi, I'm making a Death Domain Cleric/ School of Necromancy Wizard and I just had a thought. Does the spell Vampiric Touch combo with Channel Divinity: Touch of Death and Grim Harvest?

Yes.

ToD requires only that you make a melee attack. If you do the attack deals 5 + CL necro damage.

VT grants a melee attack that deal 3d6 necro damage and heals for half the damage dealt

GH requires you to kill a target with a spell of 1st or higher and if you do receive healing to 2x spell level, or 3x spell level if necro damage was dealt.

VT is 3rd. So let's say you had 2 levels in Cleric (Death) and 5 in Wizard (Necro).

You cast VT and make a melee spell attack. You roll and get 3, 2 and 5. That's 10. You channel divinity and choose Touch of Death. That's 5 + 2, or 7 extra necro damage. That's 17 total, healing half rounded down which is 8. The attack kills the target, triggering Grim Harvest. That's extra 9 healing (3rd level, x 3 as necro). So, you dealt 17 necro damage and healed 17.

It's a decent combo.