Keep in mind that you can change the gearing on your Vita by swapping out the cassette for something with closer ratios. Since you have a "granny" gear on this model, you can go with quite a close ratio and still have hill climbing gears. That's a fairly inexpensive upgrade. And it's possible that the in-between gear you need is on the other chainring.
As for getting tired quads - are you pulling back and up at the bottom and back of the pedal stroke, using your hamstring and hip muscles, or are you only pushing forward and down with the muscles on the front of your leg. If it's the latter, pedals with a bit more grip or clipless pedals can divide up the workload between your leg muscles more evenly.