I´m also looking for Mariachi style trumpets too. I heard the Roland SRJV18 Latin card which have several mariachi trumpets and sound very nice but their ranges are a little limited. I´m not so shure about the VA trumpets, may be they are the same samples.
Roland once developed a S770 series cdroms (World Instruments) which mentions in its brochure Mariachi brass, Guitarron (Mexican Acustic Bass) and Vihuela (5 string \"alto\" guitar) along with a lot of other country´s instruments. I´m wonder if anybody knows if this Cdrom ever was released by Roland.
Right now, I´m not aware of any usable trumpet sample for this style of music or even for pop in general.