I definitely believe it more now than I did before. I don't think it happens much in the MLB as in the NFL or NBA during the regular season, but still have to be skeptical.
More serious money is bet on the NFL and NBA every week from all over the world. NFL is probably the most bet sport every weekend. Also, it's the only sport that has a two-minute warning at the end of the half and at the end of the game, where the game comes to a complete stop. Why haven't people asked that question? In the NBA, all the timeouts that take place in the last two minutes of a game, even when the game is out of reach for one team. There definitely is some element of fixing the outcome of scores, over/under and games overall. Have to worry about the games that don't mean anything or the games where you have a heavy favourite.
As for the Playoffs, I think there is an element of fixing if the league wants a certain team to go through and advance. NFL and NBA referees can make calls that influence a game. As for Vegas having an impact, don't think it's as prevalent as during the regular season. Maybe with the over/under's or point spreads.