To clarify a bit for people who may be unfamiliar with Macross:
Super Dimensional Fortress Macross was released in 1982 in Japan. There was one lead male character voiced by Arihiro Hase, and two lead female characters, one of which(the more iconic one) was voiced by Mari Iijima. The 1980s was a time where US companies were licensing anime series, then rewriting the script and editing the series to get something suitable for morning action cartoon blocks for US TV. Macross was the first of three series that was used to create Robotech with an '80s dub cast. It was popular in the US which led to US popularity of the original Macross franchise as the anime scene started building up in the early 90s and Macross OVAs/movies got released. Meanwhile Mari Iijima moved to the US in 1989 in further pursuit of her music career (and as of last time anyone updated her wikipedia entry, she still lives in Los Angles, California). Since then her music career has been her main work, not voice acting.
In 2006, ADV Films got the rights to release the original SDF Macross series in the US. This of course required a new translation and a new dub as the previous one had a bunch of name changes, odd rewrites, and probably some bits that had dialogue in Japan but were cut and received no dialogue in the US, or the reverse where the US added some talking where the original had nothing. As Mari Iijima was available and willing, ADV Films cast her to reprise her role, this time in English. For the male lead they cast Vic Mignogna and for the second female lead they cast Monica Rial. I'm not certain how ADV films ran it's dubbing operation, so I have no real idea of how much contact Mari Iijima would have had with Vic or Monica for most of this process. However ADV Films took full advantage of having access to an english speaking member of the original Japanese cast, and had Mari do a bunch of stuff for the special features. Said special features included doing some episode commentary with various people,
including Monica Rial for DVD1, and
Vic Mignogna for DVD2. The DVD release finished in 2006, and such commentary generally requires being in the same studio at the same time, so Mari Iijima would have met and worked with Vic sometime in 2006.
Furthermore, the conventions they were both guests at are:
Anime Weekend Atlanta 2004
Tekkoshocon 2007
Ani-Magic 2007
Kawaii Kon 2008 (Monica Rial was also a guest)
Anime Central 2008
Tekkoshocon 2010
MechaCon 2010
A cursory search did not turn up any other conventions where both Monica and Mari were guests. Given their lead roles for a popular lead series like Macross, I would not be surprised to learn that Vic and Mari did a panel together at one or more of these conventions; although I have not looked for old convention programs or schedules to try and confirm.
So in 2007, Mari Iijima was not some random celebrity guest flown in from Japan. She was an english speaking US resident who at minimum was acquainted with Vic, had worked with him before, and would go on to be guests at the same convention as him a few more times, some of which may have involved working together with him for panels.