My two pieces of advice are this:
(1) I often start with Travelocity or Hotwire to get a feel for which rental company has the best rate for the dates I'm searching but there is usually no price advantage for booking through them. 99% of the time I will search on the travel site and then go to the company's actual site to book the rental. It just makes it easier in the end if you need to make a change, etc.
(2) I realize that a lot of people mention cheaper flight or car rental to go back to Las Vegas, however you need to consider your opportunity cost of your time while on vacation. If time is of no question and you could just assume take an extra day or two, that's fine. However, the average person usually has a fixed amount of vacation time available.
If, at the end of your DLR trip, you need to account for driving back to Las Vegas, pay for the associated gas, and account for the numerous potential travel setbacks that could occur during the 3-4hour trip, the effort, time and hassle would likely require the cost benefit to be several hundred dollars less to make it worth it for me.
Even coming from the Midwest, I have never found any flight difference between Las Vegas and Los Angeles to be worth my effort to drive the distance and take that travel time to make it worth the trip into Las Vegas for a return flight.
I would imagine that even if you found a car rental that was half the price you found, it would still not be worth it in the end to travel back to Las Vegas.
Regardless of how you decide to proceed, have a great trip. I did a Vegas/DLR combo trip a few years back and it was a blast.
Have fun!