From what I can see Vettel did make an opportunistic move. Like marvin78 says, that can work if you are up against a driver minding his own race and who realizes he is outclassed anyway. Such a driver will leave a lot more room.
Watching the replay, Verstappen was hard but not overly so on squeezing Vettel: https://www.youtube.com/watch?v=8N3SItwAvKU
Basically he left Vettel enough room to be on track. But Vettel required more room to slow the car down. I think Verstappen did fail to realize that. Verstappen is entitled to his line because he did not take defensive action before the corner, thus is allowed to squeeze him but naturally leave enough space so not to push Vettel off the track. Verstappen did the minimum and nothing more. So Verstappen is morally fine to take that line, but he wasn't smart about it either. At that point the car of Vettel is more of a projectile heading for your car, and such a thing can easily cost your own car and race. Verstappen should have known this because his PU was clipping, so he should have known he was down on power and checked his mirrors to see the speed difference, and perhaps having taking a more defensive line approaching the corner, discouraging Vettel to plunch it down the inside. Or, take a wider line. With Vettel having to slown that much in a basically mid speed corner, and a straight following, I think Verstappen would have kept the place pure because of the acceleration.
Vettel on the other hand left himself completely at the mercy of Verstappen's choice, depending on Verstappen to voluntarily leave enough room to get the car to slown sufficiently. Verstappen is not the type of driver to let anybody by easily, and Vettel should have known that. That being said, I understand for doing that. To have been any meaningful in the WDC, he had to get rid of Verstappen as soon as possible and get to the Mercedes cars. Basically Vettel had to win and had to move up. So we should appreciate Vettel's approach. It just was played against the wrong type of driver.
Here are by the way some similar incidents but with important differences:
Vettel here takes the outside line (he could no do that with Verstappen because Verstappen was already on the outside). Because Vettel shot in front of Ricciardo before the hairpin, Vettel was allowed to dictate the line. He also was much more in control, able to control the space between him and Ricciardo.
Reversed situation, where Ricciardo dives on the inside of Vettel. Vettel saw Ricciardo was going to need (a lot) more room to navigate the corner, and opened up the steering wheel. Not because he was obliged to, but because if he didn't it could have ended his race. Admittingly, Ricciardo went in way hotter that Vettel did in Japan. Vettel would have navigated the corner in Japan.
Similar incident to Vettel, but Verstappen locked up and carried way too much speed to navigate the corner properly. Vettel left more than enough space and probably did not expect, I wouldn't, an overtaking attempt from that far back.
So end conclusion: I think Vettel tried an opportunistic move, but not overly opportunistic as he would still have navigated the corner in a reasonable fashion. Given Verstappen was entitled to the racing line and also left just about a one car width room, but was not going to give anything more than that, Verstappen did nothing wrong in a technical sense. The collision was caused by Vettel requiring a bit more track and Verstappen not willing to give that. It ended up in a racing incident. I think that was the correct reasonable decision by the stewards.