I installed a LED reflector bulb in a light fixture, but the lamp says to not use in totally enclosed fixtures. The fixture I installed it in is a recessed shower light, and it has just a couple small vents. Will it harm the bulb?

It could be overheating, so expect short life...

It depends on the power: If you use a single 4W lamp in a 100W rated fixture, it would be very likely OK.
But if you use 12W LED at the same place, it may get into heat troubles...

It also depends on how long you run it for, if it's only on during showers odd are it won't get hot enough to bother it, but if you let it run all day long it might overheat, this normally effects the life of the electrolytic capacitors in the power supply, so don't expect it to last the stated hours on the box...