It's been my assumption that a laser would be an effective weapon at short enough ranges that a pulse can vaporize hull plating.
"Short range" is a tricky thing to quantify. It very much depends on your own tech level assumptions and requirements, and as you haven't communicated them to us then I can't really speculate. The shorter the wavelength, the fancier your optics, the longer range your lasers. You more or less get to handwave this to suit your own purposes, and pick something between hundreds of km and lightseconds.
A bomb-pumped laser, for example, might have a range as little as 100km (which is more like practical casaba-howitzer range) or as long as 10000km, depending on the quality and efficiency of your lasing rods, and the effectiveness of your targetting system.
Beyond that, it was my assumption that a laser would be fairly ineffective, since it would only be able to heat the target.
Being gently warmed by x-rays may not be quite the same as being gently warmed by a heat lamp. Just sayin' ;-)
I remember hearing somewhere that something like 20% of the energy put into a laser is lost as waste heat,
An 80% efficient laser isn't implausible, but might well be considered extremely efficient. There are some diode laser designs that reach that level of efficiency, but many other kinds of laser, especially very short wavelength or very short pulse designs will have lower efficiency. (The bomb pumped lasers I mentioned above might only be 2% efficient, but when you're pumping them with a megaton bomb and you're using a thousand of them, the inefficiency isn't so much of an issue...)
Can I use reflective hulls as a defense against lasers at long range?
There will be some range where the incoming beam is juuuuuust intense enough to melt your armour, but if you buffed it a bit first then you'll be fine. This depends on the beam actually being reflectable, of course... at the far-UV and shorter wavelength range (eg. bomb-pumped x-ray laser) it becomes a bit difficult to do this, because your electrons will start falling off, but for visible light you might be able to get away with it.
Lasers, by definition, operate at a single wavelength
Generally, yes. It is possible to adjust the wavelength of a free electron laser by a small amount, limited by the available optics. A visible-light FEL might be quite adjustable, for example. It is also possible to carry more than one laser, or a laser which can be frequency-doubled (in the way that current-day commercial near-IR Nd:YAG lasers can use a KDP crystal to produce visible green light, at some efficiency cost). Ti-Sapphire lasers can have their output tuned to some extent, between 650 and 1100nm.
so as long as the material reflects that specific wavelength, I would assume that they would.
You can make highly efficient dielectric mirrors, but they tend to be efficient in a very specific wavelength band. They're expensive (much more so thanb dumb armour) and delicate (abrasion caused by micrometeorite damage will render them useless) and will be largely ineffective against any laser light of an inappropriate wavelength.
Moreover, at effective killing range you'll start to get very high laser intensities and interesting non-linear optical effects, which means that reflecting 99.99% of the incident light just isn't enough and your mirror surface will blow up in a nanosecond and be useless.
Oh, and buttering you up with a few bomb-pumped lasers at longer range might be enough to render your armour ineffective against more conventional laser fire.
For these reasons, they're probably not a good choice for warship armour.
In other words, will wallpapering my ship with aluminum foil help?
It'll help, inasmuch as putting stuff between you and their laser will keep you alive for a tiny bit longer when things start going south. Mostly what you want is something highly refractory, because you'll want your armour to still be useful at closer ranges when you simply can't reflect enough of the heat to be safe and need something that will absorb as much energy as possible before evaporating.
Some form of carbon is probably best. Boron will also work, but it is much rarer and you may find there are other, better uses for it. A weave of carbon nanotubes is probably the best thing to use here, as it'll resist being torn open by explosive effects. In any case, these materials will also be useful against other kinds of energy weapon. Maybe some harder inclusions (some form of hyperdiamond, perhaps) might help against kinetic energy weapons, inasmuch as anything helps.