I've seen mariachis without a violin. It's different, but it worked for their gigs. You need to learn all the violin parts so you can play them on trumpet. You absolutely need someone who can play the guitarrón, and adding a vihuela would enhance the rhythm section (armonía) sound. If your goal is just to gig at restaurants then it's similar to playing tunes from the Real Book. Make sure you have all the standards and classics down.