Do I put the pot roast above or below the vegetables in my crock pot?
Either way should work, though the results might depend on your crockpot's cooking temperatures.  ("High" and "low" can create different temperatures on different models.)

It comes down to your personal preferences about:


How well-done or "mushy" you want the vegetables to be
Whether you want the vegetables to cook in the meat juices and get the full "meaty" flavor
Whether you want the meat to be "basted" by the flavor of some vegetables 


In general, if you place vegetables on the bottom, hard vegetables like carrots and potatoes will get softer faster, both due to the heat and ultimately being submerged in any juices from the roast.  If you cook them very long, they may get quite mushy, which some people like and others don't.

If you place the vegetables on top, some of them may stay firmer, but they often won't absorb as much of the juices from the meat (and any flavors that go with it).  On the other hand, aromatic vegetables on top will contribute their own juice to the roast's flavor, so (for example) putting onion slices or quarters on top will allow some of the flavor to seep down gradually onto the meat as it cooks.

Some people find that even with the vegetables on top or on the sides, they still get too mushy for their tastes.  In that case, you may want to add the vegetables for the last few hours of cooking only.  On the other hand, some people really prefer the mushy texture, and in some crockpots that cook at a lower temperature the steam will not be enough to soften some of the hard vegetables (like carrots) even over many hours on top.