Good point. Although i dont think that many publishers will give discount when buying the same game on two platforms. It would be a good idea though, i agree with that.
some companies can and some cant. Like how assassin's creed games price drops pretty fast after launch for the sake of generating more sales. A pack can/will significantly reduce their profit. However, they can regain that with enough sales. Not to mention that porting it to vitas isnt hard or convoluted. That is of course assuming their framework isnt fucked up or their code isnt messy or they are competent (*points at capcom*)