I rarely use SK while grinding. Vamp is used on the cooldown, and you should only worry about losing hp when you amb sac (keep barrier up 100% of the time).
Of course you can't do that as well before lvl 37 or so but vamp should be draining enough hp for you not to worry too much about resting.
When I grind, I usually max mental for arcane missile and blaze, get lvl 4 vamp (sk too for emergencies) and get meteor, stalagmite and crystal blast to lvl 4. You can do that at lvl 38 or 39. It's the most mana efficient method of grinding I've seen before, and it is affected less by resists due to the low cooldowns of arcane missile and blaze.
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