Necromancer: leech on hit and leech on crit scaling

Looking into the two specific abilities you've mentioned, I came across this page on the Guild Wars 2 wiki dealing specifically with life stealing, which mentions the following two things:

Siphon Health from Vampiric is capped at 25 base, it gets to 38 with Bloodthirst. It doesn't scale with power or healing power.

This indicates that the Vampiric ability does not scale with healing power, and is capped at a specific value, so does not scale with damage done either.

Siphon Health from Vampiric Precision heals for 31 (without Healing power)

This indicates that Vampiric Precision does scale with healing power, but doesn't mention the scale of which healing power affects this ability.

Both of these abilities are affected by the Bloodthirst ability.