Most - quite possibly all - of the missions that are active churches require that at least one of the couple being married to be Catholic. You usually have the opportunity to bring your own priest.
You may have to consider one of the missions which is not an active Catholic church (the nearest, I believe,is La Purisima in Lompoc which wouldn't be very convenient) or find a nearaby venue. There is an inviting park across from San Fernando, Brand Park at 15174 San Fernando Mission Boulevard. You might call the rectory and get their advice (818-361-0186). If they are unable to hold the service they may be able to give you some additional suggestions.
One other idea would be to find a wedding photographer who covers this area and explore venues with them.