Mustang, I actually live in Vista, so I know where you might go fly. Honestly, there's not a lot of good areas to fly, at least, not safely, in parks/open areas; most of that is due to the airspace around being either restricted by the military (Camp Pendleton airspace) or being in the flight path of Palomar Airport or Oceanside Airport. In addition, people are freaked out by drones and spying, so they tend to flip out if you fly at a park...
There are a few guys who fly drones at Brengle Terrace in Vista on the weekends, but a lot of the time you're having to dodge disc golfers, so it's not really ideal. Pretty much everywhere else is taken up with soccer or baseball, and the Oceanside airport and Palomar Airport put a kibosh on a lot of flight, as does Pendleton airspace.
That said, there is slope soaring that happens on Wednesday afternoons off the coast of Carlsbad, but you have to keep your gliders low. If you're looking for a good field, Johnson Field (part of the Palomar RC Flyers) has a GREAT field - there's a runway for fixed wing planes, as well as an open area for the helis, and a small rolling hillside area for drone flights. The field's right off the 76 and 15 freeway, and they're open to just about anything for flying. I know I tried to look into the San Diego Wingmasters and their field off of Twin Oaks Valley Rd., but the landlord of the field does not allow any rotary wing (i.e. helicopters, autogyros, paragliders, tri/quad/hexa/sexta/septacopters) aircraft to be flown there, so I ended up joining Palomar R/C Flyers.
If you do come out, mention that Doug Abel, the guy who owns the Sea Duck, is the one that told you about the field.