The merger and name change happened the year after I graduated undergrad from UTPA. I don't know the details, but something about how making the medical school happen would be easier if the two universities merged into one "new school".
When the Vaquero name first came out, I think it was almost universally hated by those on campus. Why? The powers that be actually held a survey for students and alumni to vote on a mascot. They then completely ignored the results and settled on Vaqueros. I believe "Phoenix" was the most popular mascot choice followed by retaining Broncs. It really felt like it was some old white dudes trying too hard to be cultural and settling for the first Spanish word that made sense. Students did not like it. Feelings might be different now as new students are coming in that weren't attached to the Bronc mascot and weren't a part of that process.
Speaking of stadiums, being from Weslaco, I'm partial to Bobby Lackey Stadium. Seats just shy of 15,000 I think with some nice press facilities, new turf, and a great scoreboard and media team. Easy access right off of the expressway. It is about 30 minutes from the Edinburg campus though.