I take it you didn't watch game 1. Or much of Vegas' last four months of hockey.
Starting with goaltending, Hill struggled with injuries for a good chunk of the year but when he first came back people were talking about how the only reason he wouldn't be a Vezina candidate was because he hadn't played enough games and people were starting to have hope team Canada could have a strong starter at the Olympics with him in net. After that his performance and confidence tanked to the point that Thompson stole the starting job back after losing it last year. Thompson mostly played well but had some big stinkers like the blowout loss to Arizona and I'd argue two of his goals against last night were incredibly weak, though I can't argue that overall 26 saves on 29 shots was solid.
Defensively, the Knights haven't been anywhere close to the 5 man defensive structure, pressure, and ability to win body/positioning/puck battles they were at last year. That was something that started to develop around January of last year and kept improving right up to the cup win. This was pretty evident last night as Vegas let Dallas come in with some really dangerous unmarked chances in the first two periods before clamping down with turtle hockey. There's been no real improvement to the defensive structure and effort compared to the past and the chances of getting back to that level are pretty slim.
Overall the offense is just not there. Last year Vegas didn't exactly play the high octane kind of relentless offense that Edmonton plays with but they were still pounding away at opposing defenses almost every shift until mistakes led to chances. This year, from game 1 the Knights have played this sort of reserved style of offense that's very focused on limiting errors, looking for optimal plays, and waiting for errors to occur rather than forcing them to. Being limited to 15 shots on goal last night wasn't a surprise to me. It's exactly in line with a continuing issue that hasn't gotten better over time and they were only fortunate that Oettinger has a weak night.
Either way, it's no accident that Vegas ended up as the 8 seed. Throwing in a couple big names on a roster sheet doesn't make ongoing flaws just vanish.