Haha... good logic and it works but lets assume that they didn't drive, they took the plan to Vegas.
A trip in this case is a vacation. To make it clear, lets assumed Vern and Shirley flew to Vegas for a week long vacation.
Haha.. come on now. It's not that hard.
When you guys give up, I'll share the answer. 
You are changing the scenerio. This is how it went; Shirley had a previous husband name Vern 1, who she is now divorced to and they went to Vegas, which is one trip each Shirley and Vern 1. She married another husband who's name also happened to be Vern too; Vern 2. They take a trip to Vegas; two trips for Shirley (with the two Vern husbands; Vern 1 and Vern 2), while it is only one trip for Vern 2.
If this is not the right answer, then the only scenerio is that their plane crashed right at the border; Vern's body tumble right into Vegas, while Shirley's body tumbles into Vegas, bounced back to Cali across the border, then ricocheted back to Vegas again.....LOL.