It seems to me that the PSP (not the Vita) is being neglected on the PSN, this includes the Plus content. Everything is fpr the PS3 and the Vita now. When is the PSN going to put any free content on plus for the PSP that is worth having? Since I signed up for Plus, it has been alot of PSNeglect.
Whatever "PSP" support you are going to get will be in the form of PS Minis and PSone classics. PS+ was primarily meant for PS3 owners, if you also own a PSP then that is a bonus, but it was never intended for PSP only games to be offered on PS+. Whatever products on PS+ that run on PSP, will also run on the PS3 which is their primary focus.
I do expect Sony to include the Vita as part of PS+ soon, since they are actively trying to push that platform. Unfortunately the PSP has hit the end of the road..