Seth, your inspector gave you the basics, not detailed instructions. But that's not his job. There is a new movement afoot, at least in the South, that contends that we don't need ventilation under a crawlsapce, if it's sealed properly. The "theory" is that if there's plastic, sealed around the edges, keeping moisture from entering through the ground and there's blown, compressed cellulose insulation on the perimeter walls then there won't be a mosisture problem because there won't be any moisture in or entering the crawlspace. Insulation on the perimeter walls can't be "tacked" up. It must form a seal around the perimeter wall and with the plastic on the ground. Floors need to be insulated. Of all the places in a house that lose heat, the floors lose the least. Of course, there are exceptions to that.
Having been an inspector for around 13 years (bad number), I can say that we normally pass on a lot of information on things that could be done but don't necessarily have to be done. And a lot of the time, it causes undue concern and worry to the buyer, because a lot of time, since we mentioned it, they think that they need to do it. I found a tree growing about 4 feet from a house once. I mentioned to the buyer that we didn't recommend any trees within 10 feet of the house and the next thing I know he's trying to get the seller to cut the tree down before he buys the house. It had only been growing there for 15 or 20 years and hadn't caused a problem yet. Some things like insulating the crawlspace, are just nice to do things that could save you some money on your heating and cooling bills. However, with the little bit of savings that you'll receive, you may have sold and moved on by the time you reach a break even point in relation to the cost. If you're not having moisture problems, mold, mildew, etc. then proper ventilation under the house should be sufficient. Most building codes even state that you should have x square feet of ventilation per 100 sq ft of floor space. If you're going to sell VA or FHA then you will probably have to insulate the perimeter walls before selling.
Covering the vents, thereby stopping the escape of moisture, will cause problems unless the crawlspace has been prepared properly. Not easily done after the house is built. If you're not having problems, I'd vote for leaving well enough alone. Spend the money on attic insulation, caulking and updated heating and air system. Much quicker return on your money and more comfort too.