Yeah, so far Pence's only problem seems to be he lied about what he knew, but that happened in media interviews, not under oath, so there's no perjury case.
Pence's main mission in all of this (which he seems to have failed) was to keep himself clear of the stink of Trump and his associates so that if the worst came to pass and Trump needed to be removed, he could step in relatively clean and the furor would die down. He was never in any real legal jeopardy though.
I bet Mueller is looking into why Pence lied about what he knew and when he knew it.