For a more flat route, but a waste of time since you already crossed the I-80.
Head East out of Vernal through Dinosaur and go toward Craig Colorado. From Craig, head North to I-80 in Wyoming, then head East on I-80.
If you can deal with 2 more large hills, much more scenic route.
Go on through Craig to Steamboat, and tie in to I-70 at Silverthorne. This will put you on the West side of the Eisenhower Tunnel (big hill number 2), then go on to Denver. It's flat after that. I didn't bat an eye at the climb out of Steamboat, but I was on a motorcycle; I wouldn't hesitate to run my truck/trailer on this route (would prefer it vs the boring I-80), but I've got plenty of power on tap.