I've yet to see a report that shows strong evidence that phone/tablet games reduce core game sales on other platforms.
I'm sure there is some impact, but it's unclear what it is. The same is true of Freemium in the PC Space FWIW.
I'm sure phone gaming disuades some people from having an additional portable device purely for gaming, but I have no idea how you would quantify it. There are plenty of core reasons Vita is selling poorly ATM, a lack of compelling content being one, price another.
Uncharted or LBP or Gravity Rush on Vita have no equivalents in the phone or tablet world, I don't know that people compare prices between the formats. It'll be interesting to see if Vita picks up in sales towards Xmas when there is more content available. It's not like PSP flew off the shelves early on either.