Vader's appearance will likely be an epic fail......so many ideas thoughts how he "should be" that anything else will likely result in critisim.
Personally, I'm not holding out for much screen time so anything different will be fine in my book....melo, kick ass, cool etc.
It's been confirmed he has spoken lines but that could just be a few tag lines at the end.
The real proof will be in the trailer that plays with "Dr Strange" (so in a few weeks) - if that doesn't show a hint of Vader in action, then it probably is just a cameo; walk in, say a few lines, and fade out.