Battery vampire for Eneloops bad idea?

I'm wondering if using a battery vampire with Eneloops would be harmful to them? I was thinking about getting the most use out of them, but then am wondering if that would be harmful or hard to charge?

I'm not sure I understand your reasoning.

Generally the idea behind a "battery vampire" is to get every last erg of energy out of disposables before you discard them. Thus you save money and reduce waste. Eneloops are rechargeable so this concept of draining them to dust before disposal doesn't really apply.

But aside from that, total drain of rechargeable is, almost universally, hard on the cells and will generally result in a shorter duty cycle.

Many battery vampires are not actually very efficient. Their specialty is in working down to near 0.2V. This becomes doubly a problem with Eneloops, since there is very little usable energy below 1.0v per cell, reachable by most (high-power, 85% efficient) boost drivers. Draining them to 0.0v is not especially good for even quality NiMhs, and seems to increase internal resistance. Low-quality NiMHs suffer worse.

If you want a "neat" light, stick two Eneloops (Or any 1.5v-ish cell) in series with a white LED. This will provide weeks of continuous light at almost-100% electrical efficiency: Mini Mag Lite Forever (The only power 'lost' to drive is that in the cells' internal resistance and the mag switch and body).

Short answer: Bad idea.

Discharging too low is bad for the cell. To get the most use out of them like you want...recharge them and use them. They'll do fine in something that gives very long run time.