Yes! Read your recipe to see what it says to do. Now, a roast you can put in, cover with water, pack of italian dressing or french onion soup and let it go on high all day and it will be fine. Just research it a little and you will be amazed at what you can cook in them.
The answer is yes! I frequently do a pot roast with vegetables in the slow cooker- so I recommend browning the meat in a skillet, putting it and the broth of choice to slow cook all day or night. Then I add the potatos, carrots, whole garlic and onions in for the last hour or two. Otherwise the veggies turn to mush. Season the meat well and you won't even notice the veggies weren't in there all day.
Good luck, you will love it! In the grocery store I found crockpot liners, similar to the roasting bags- pricey, but cleanup is sooooo easy. And there are zillions of recipes online.
Let us know how it goes or if you have any problems.