Do veep picks ever bring states into play that other wise wouldn't have been?
NO.
Yes, they do.
Muskie brought Maine to Humphrey which Nixon would otherwise have carried.
Gore might well have tipped Tennessee to Clinton in 1996.
Bricker tipped Ohio to Dewey in 1944.
I think Stevenson brought Illinois to Cleveland in 1892.
There is absolutely no evidence to suggest that any of those victories were directly related to the veep pick.
Why else would Maine have gone so solidy for Humphrey?
I don't know much about either Maine or the 1968 Presidential Election. My best guess is that it was the beginning of realignment. Massive landslides in 64 and 72 made it hard to gauge, perhaps it was the result of continued distaste with the post-Goldwater GOP (Johnson won Maine by 37 points)?