you're getting married at the Westin Los Cabos right?
So am I, so I can serve as a bit of an authority. YES you ABSOLUTELY can bring all your own vendors (except for food I believe). I am obsessive about photography, so I booked my photographer first after confirming my date with the Westin- Bradley Fraser. One thing I will warn you about is that the Westin will try to screw you by charging fees for bringing your own vendors. There is a small fee for the unions that they can charge, but they sent a BIG fee on our contract and my FMIL refused to pay it so they backed off. They will also charge you a corking fee if you wish to bring your own alcohol, which we are, but we aren't paying the fee because FMIL fought that one too. As long as you really bitch about it and tell them no, they seem to back off on the extra fees.
We arent getting married on the beach and havent chosen a DJ yet, so I can't help you with the sound system. My wedding is in August so I'm eager to get stuff pinned down there too. Send me a PM and we can exchange email addresses and I will give you all the info that I have!