Yuffie and Vincent were not even characters that you had to get so it wouldn't of made much sense for them to wrap an entire mission around them.
When I played FF8 I played it soon after I got finished with ff7 so it was hard for me to appreciate Selphie's character because her personality type seemed almost identical to Yuffie's. And in my opinion ff7 had far supperior character development then ff8 so whenever I saw Selphie on the screen I was thinking of Yuffie's character and not Selphie.